Steel plate excellent in machineability and in toughness and weldability and method of production of the same

ABSTRACT

The present invention provides steel plate excellent in machineability and in toughness and weldability and a method of production of the same. Steel with C, Si, Mn, P, S, Al, and N limited to predetermined ranges and further containing, as necessary, Mo, Cr, Nb, Ti, V, Cu, Ni, B, REM, Ca, Zr, or Mg is further strictly defined in the balance of steel ingredients and strictly controlled in the conditions of rolling, water cooling, etc. in the method of production to obtain steel plate where, when the plate thickness is 4 mm to less than 10 mm, the ferrite fraction of locations exactly ¼ and ¾ of the plate thickness inside from a top surface of the steel plate is 30% to 90% and a ferrite fraction of a location exactly ½ of the plate thickness inside from a top surface of the steel plate surface is 20% to 90% and, when the plate thickness is 10 mm to 100 mm, a ferrite fraction of a location 2 mm inside from a front and rear surface of the steel plate is 30% to 90% and a ferrite fraction of locations exactly ¼, ½, and ¾ of the plate thickness inside from a top surface of the steel plate surface is 20% to 90%.

TECHNICAL FIELD

The present invention relates to steel plate excellent in machineability and in toughness and weldability, in particular relates to steel plate having a plate thickness of 4 to 100 mm or so and a tensile strength of 570 to 720 MPa or so and a method of production of the same. The steel plate produced by this method of production can be used for ships, bridges, buildings, marine structures, pressure vessels, line pipe, and other welded structures in general, in particular is effective in use if fields requiring drilling, surface machining, and other machining work when fabricating a structure.

BACKGROUND ART

As steel plate used for a welded structure, high strength and also, as weldability, the suppression of weld fractures and a high weld heat affected zone toughness are usually required. In a steel material with a tensile strength of 570 MPa or more, it has been possible to achieve both high strength and weldability by keeping the amount of addition of alloy elements to a minimum and converting the main structure forming the steel bainite or martensite. However, when fabricating a building, bridge, ship, or other structure, drilling, surface machining, and other machining processes are involved. When bainite or martensite forms the main structure, the productivity of the work falls due to the increased frequency of replacement or resharpening of tools along with tool life, the drop in the machining speed due to the increase in machining resistance, etc. and, as a result, the structure increases in fabrication costs. For example, Japanese Unexamined Patent Publication No. 9-310117 tries to achieve both a high strength and weldability with a relatively low alloy elements by making the structure mainly bainite. However, since the steel has a structure mainly comprised of hard bainite, the machineability is poor and the cost required for machining work is high.

In a welded structure, the steel plate used for the main parts, for example, the webs and flanges, are welded and are subject to relatively large stress at the time of use, so require excellent weldability and toughness. On the other hand, there are parts which are not welded and parts where a high toughness is not required. For example, the tie plates etc. used when bolting main structures of bridges desirably have a toughness of a performance of an extent satisfying the standards, but do not have to have a level of the same extent as the main structures. If using steel plate having bainite or martensite as the main structures for these parts, since the machineability is poor, the machining work takes time so the structure greatly increases in fabrication costs.

To improve the machineability, in particular to extend the tool life and reduce the machining resistance, it is known that addition of S is effective. However, when adding a large amount of S, the matrix toughness falls and the weldability falls. As opposed to this, a technique for achieving both an improvement of the machineability by the addition of S and securing of the weldability is disclosed in Japanese Unexamined Patent Publication No. 6-184695. However, the weldability secured here only eliminates preheating and suppresses weld fractures. The welded zone toughness and the matrix toughness are low. Therefore, the use of such steel for welded structures is not possible. Further, a technique for achieving both machineability and matrix toughness is disclosed in Japanese Unexamined Patent Publication No. 2000-87179. By controlling the form of the sulfides by addition of Ca and Mg, the anisotropy of the matrix toughness is improved, but the absolute value is low and further the weldability is poor, so the use of such steel for a welded structure is not possible.

The machineability also depends on the configuration of the microstructure. It is known that rather than a structure mainly comprised of bainite or martensite, a ferrite and pearlite or a ferrite and bainite structure is excellent. For example, Japanese Unexamined Patent Publication No. 7-54100, Japanese Unexamined Patent Publication No. 7-109518, and Japanese Unexamined Patent Publication No. 7-166235 disclose steels with ferrite and bainite structures. Further, Japanese Unexamined Patent Publication No. 2000-63988, Japanese Unexamined Patent Publication No. 2000-63989, Japanese Unexamined Patent Publication No. 2000-282172, and Japanese Unexamined Patent Publication No. 2001-214241 disclose steels with prescribed fractions of ferrite. Steel plate with microstructures of ferrite and bainite and steel plates securing certain ferrite fractions are qualitatively excellent in machineability to steel mainly comprised of bainite or martensite, but the absolute margin of improvement cannot be said to be sufficient enough to improve the productivity in drilling or surface machining in the process of fabrication of a welded structure. Further, said art all call for large amounts of addition of alloy elements, so the toughness and weldability are low and the use of such steel for welded structures is not possible. From the above, it is not possible with the current art to produce steel plate having a 570 MPa or higher tensile strength and a high toughness, weldability, and machineability.

DISCLOSURE OF THE INVENTION

The present invention provides steel plate excellent in machineability and in toughness and weldability or extremely excellent in machineability and having a toughness of an extent enabling application to welded structures, having a plate thickness of 4 to 100 mm or so, and having a tensile strength of a level of 570 to 720 MPa or so and a method of production of the same. It has as its gist the following:

(1) Steel plate excellent in machineability and in toughness and weldability characterized in that the steel comprises, by mass %, a steel composition comprising:

-   -   C, 0.005 to 0.2%,     -   Si: 0.01 to 1%,     -   Mn: 0.01 to 2%,     -   P: 0.02% or less,     -   S: 0.035% or less     -   Al: 0.001 to 0.1%     -   N, 0.01% or less, and     -   the balance of iron and unavoidable impurities,     -   X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is         0.24 or less,     -   when the plate thickness is 4 mm to less than 10 mm, a ferrite         fraction of locations exactly ¼ and ¾ of the plate thickness         inside from a top surface of the steel plate is 30% to 90% and a         ferrite fraction of a location exactly ½ of the plate thickness         inside from a top surface of the steel plate surface is 20% to         90%, and     -   when the plate thickness is 10 mm to less than 100 mm, a ferrite         fraction of a location 2 mm inside from a top and rear surface         of the steel plate is 30% to 90% and a ferrite fraction of         locations exactly ¼, ½, and ¾ of the plate thickness inside from         a top surface of the steel plate surface is 20% to 90%.

(2) Steel plate excellent in machineability and in toughness and weldability, characterized in that the steel comprises, by mass %, a steel composition as set forth in (1) wherein:

-   -   Mn: 0.01 to 1.4%,     -   S: 0.01% or less,

X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less,

-   -   X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is 0.15 to 10.0,     -   the structure forming the steel is a structure having a ferrite         fraction of 30% to 90% and the balance of a hard structure         mainly comprised of bainite and martensite or a structure having         a ratio with a micro Vickers hardness of 190 HV or less of 20%         or more, and     -   the steel has a Vickers hardness of 165 HV to 300 HV.

(3) Steel plate excellent in machineability and in toughness and weldability, characterized in that the steel comprises, by mass %, a steel composition as set forth in (1) wherein:

-   -   Mn: 0.01 to 1.4%,     -   S: over 0.01% to 0.035%,     -   X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is         0.24 or less,     -   X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is 0.15 to 10.0,     -   the structure forming the steel is a structure having a ferrite         fraction of 30% to 90% and the balance of a hard structure         mainly comprised of bainite and martensite or a structure having         a ratio with a micro Vickers hardness of 190 HV or less of 20%         or more, and     -   the steel has a Vickers hardness of 165 HV to 300 HV.

(4) Steel plate excellent in machineability and in toughness and weldability as set forth in any of (1) to (3), characterized in that said steel further comprises, by mass %, one or more of:

-   -   Mo: 0.01 to 1%,     -   Cr: 0.01 to 1%,     -   Nb: 0.001 to 0.1%,     -   Ti: 0.001 to 0.1%,     -   V: 0.001 to 0.1%,     -   Cu: 0.005 to 1%,     -   Ni: 0.01 to 2%,     -   B: 0.0002 to 0.005%,     -   REM: 0.0005 to 0.1%,     -   Ca: 0.0005 to 0.02%,     -   Zr: 0.0005 to 0.02%, and     -   Mg: 0.0005 to 0.02%

(5) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (1) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V /10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T1(° C.) expressed by T1=35 ln(X2/2)−25√t+1070, X2=(Si/5+Mo+Cr/2)/Mn, to 720° C. and a total reduction rate of 30% to 95%, starting water cooling after the end of the rolling at a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.

(6) A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in (5), characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.

(7) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (1) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it, starting water cooling when the steel plate surface temperature is T2(° C.) expressed by T2=910−310×C−80×Mn−20×Cu−15×Cr−55×Ni−80×Mo+0.0006t²−0.56t−8.6 to 650° C. by a flow rate of 0=2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 500° C. or less, where t is the plate thickness.

(8) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (1) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, air cooling to 500° C. or less, reheating the steel plate to a T3(° C.) expressed by T3=0.0017t²+0.17t+730 to 850° C., then starting the water cooling, and ending the water cooling at 500° C. or less, where t is the plate thickness.

(9) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (2) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T4(° C.) expressed by T4=35 ln(X2/2)-251t+1100 to an Ar₃ point by a total reduction rate of 30% to 95%, then speedily starting water cooling after the end of the rolling at a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.

(10) A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in (9), characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.

(11) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (2) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rolling it, cooling it to 500° C. or less, reheating the steel plate to 900° C. to 1050° C., water cooling it by an average cooling rate of 1° C./s to 100° C./s, and ending the water cooling at 500° C. or less.

(12) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (2) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to an Ar₃ point to a temperature lower than the Ar₃ point by 150° C., starting water cooling at a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 500° C. or less.

(13) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (2) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or Less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to 500° C. or less, reheating the steel plate to 730° C. to less than 900° C., then water cooling it, and ending the water cooling to 500° C. or less.

(14) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (3) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T4(° C.) expressed by T4=35 ln(X2/2)−25√t+1100 to an Ar₃ point by a total reduction rate of 30% to 95%, then speedily starting water cooling at a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.

(15) A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in (14), characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.

(16) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (3) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rolling it, cooling it to 500° C. or less, reheating the steel plate to 900° C. to 1050° C., water cooling it by an average cooling rate of 1° C./s to 100° C./s, and ending the water cooling at 500° C. or less.

(17) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (3) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to an Ar₃ point to a temperature lower than the Ar₃ point by 150° C., starting water cooling at a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 500° C. or less.

(18) A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in (3) and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then cooling it to 500° C. or less, reheating the steel plate to 730° C. to 900° C., water cooling it, and ending the water cooling at 500° C. or less.

(19) A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in any one of (5) to (18) characterized in that said steel slab or cast slab further contains, by mass %, one or more of:

-   -   Mo: 0.01 to 1%,     -   Cr: 0.01 to 1%,     -   Nb: 0.001 to 0.1%,     -   Ti: 0.001 to 0.1%,     -   V: 0.001 to 0.1%,     -   Cu: 0.005 to 1%,     -   Ni: 0.01 to 2%,     -   B: 0.0002 to 0.005%,     -   REM: 0.0005 to 0.1%,     -   Ca: 0.0005 to 0.02%,     -   Zr: 0.0005 to 0.02%, and     -   Mg: 0.0005 to 0.02%

According to the present invention, by making the structure forming the steel a composite structure of soft ferrite and hard bainite and martensite and [1] strictly defining the ferrite fraction of the front and rear surfaces of the steel plate having a particularly great effect on tool wear, [2] adjusting the balance of steel ingredients so as to enable a great reduction of the machining resistance occurring at the time of machining at a high temperature, or [3] further raising the amount of addition of S in a range not causing a drop in toughness, it becomes possible to provide steel plate provided with a machineability of a high level not achievable with steel plate for welded structures in the past, excellent in strength, toughness, and weldability, and having a tensile strength of 570 to 720 MPa or so and a method of production of the same. The invention has high industrial value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for explaining the locations for measurement of the micro Vickers hardness prescribed in the present invention.

BEST MODE FOR WORKING THE INVENTION

The present invention will be explained in detail below.

The inventors engaged in intensive studies on a method of production of steel plate described in (1) of the present invention having a plate thickness of 4 to 100 mm or so, a matrix strength of 570 to 720 MPa or so, and excellent in matrix toughness, weldability, and machineability. As a result, they discovered that by making a composite structure of a soft structure mainly comprised of ferrite and a hard structure mainly comprised of bainite and martensite the main structure of the steel, strictly defining the ferrite fraction of the front and rear surfaces of the steel plate having a large effect on tool wear since it corresponds to the start and end point of the machining work, strictly defining the method of production requiring water cooling, etc., the machineability is greatly improved while securing the strength and matrix toughness and weldability.

Note that the “weldability” referred to in present invention indicates both weld fractures and weld heat affected zone toughness. The more difficult the occurrence of weld fractures or the higher the weld heat affected zone toughness, the better the weldability. On the other hand, the “machineability” indicates the tool life, machining resistance, and chip control. The longer the tool life, the lower the machining resistance, and the easier the chip control, the better the machineability.

The most important thing in the steel plate described in (1) of the present invention is the formation of a large amount of ferrite near the surface of the steel plate. In the machining process, the surfaces of the steel plate correspond to the start and end points. A large load is applied to the tool, so the machining in these regions has an extremely large effect on the tool life or machining resistance and chip control at the subsequent machining. That is, by making the structure at the top and rear surfaces of the steel plate a composite structure of a soft structure and hard structure, the soft part easily deforms at the time of starting and time of ending the machining of the workpiece by the tool, while stress concentrates near the interface between the soft part and hard part. As a result, the machining is started and ended by extremely small plastic deformation. Due to this, the tool life becomes longer, the machining resistance falls, and chip control becomes easy. The inventors evaluated the machineability for various steel plate with different distributions of ferrite in the plate thickness direction and discovered that it is desirable to define as values representing the distribution of ferrite in the plate thickness direction, in the case of steel plate having a plate thickness of 4 mm to less than 10 mm, the three locations of the locations exactly ¼, ½, and ¾ of the plate thickness inside the steel plate from the top surface of the steel plate (hereinafter called the “t/4 part”, “t/2 part”, and “3t/4 part”) and, in the case of steel plate having a plate thickness of 10 mm to 100 mm, the five locations of the locations of the above three locations plus the locations exactly 2 mm inside the steel plate from the top surface and rear surface of the steel plate surface (hereinafter called the “top surface 2 mm part” and “rear surface 2 mm part”).

In the case of steel plate having a plate thickness of 4 mm to less than 10 mm, the inventors discovered that when the t/4 part and 3t/4 part have a ferrite fraction of 30% or more and the t/2 part has a ferrite fraction of 20% or more, the machineability becomes good. On the other hand, when even one of the t/4 part, t/2 part, and 3t/4 part has a ferrite fraction over 90%, the strength greatly falls. From this, the inventors defined the ferrite fraction for the t/4 part and 3t/4 part as 30% to 90% and the ferrite fraction for the t/2 part as 20% to 90% for steel plate having a plate thickness of 4 mm to less than 10 mm. Note that when the t/4 part, t/2 part, and 3t/4 part have ferrite fractions of 50% or more, the machineability is greatly improved, so preferably the t/4 part, t/2 part, and 3t/4 part have ferrite fractions defined as 50% to 90%.

In the case of steel plate having a plate thickness of 10 mm to less than 100 mm, when the top surface 2 mm part and the rear surface 2 mm part have ferrite fractions of 30% or more and the t/4 part, t/2 part, and 3t/4 part have ferrite fractions of 20% or more, the machineability becomes good. On the other hand, if even one of the top surface 2 mm part, rear surface 2 mm part, t/4 part, t/2 part, or 3t/4 part has a ferrite fraction over 90%, the strength greatly falls. From this, the inventors defined the ferrite fraction of the top surface 2 mm part and rear surface 2 mm part as 30% to 90% or less and the ferrite fraction of the t/4 part, t/2 part, and 3t/4 part as 20% to 90% for steel plate having a plate thickness of 10 mm to less than 100 mm. Note that when the top surface 2 mm part, rear surface 2 mm part, t/4 part, t/2 part, and 3t/4 part have a ferrite fraction of 50% or more, the machineability is greatly improved, so preferably the top surface 2 mm part, rear surface 2 mm part, t/4 part, t/2 part, and 3t/4 part have a ferrite fraction of 50% to 90%.

Here, the method of measurement of the ferrite fraction is defined. The measurement is conducted for the plane parallel to both the rolling direction and plate thickness direction (hereinafter called the “L plane”). Avoiding the ends of the steel plate in the width direction and the spans of locations inside the ends in the width directions by exactly the lengths corresponding to the plate thickness, test pieces of the entire thicknesses were taken from locations near the center part of the steel plate in the width direction as much as possible, polished at the L planes, and etched by Nital. After this, the L planes were observed under an optical microscope. A magnification of 500× is preferable. Observation was performed using an eyepiece with a net pattern. The number of lattice points corresponding to ferrite were counted. The fraction (percentage) of lattice points corresponding to ferrite in all of the Lattice points is defined as the ferrite fraction. The measurement was conducted for a minimum of 10 fields for each location. The amount of displacement from one field to the next field was held constant. Here, when a location was difficult to judge as being ferrite or another phase, it was counted as 0.5. Note that regarding the criteria for judgment of ferrite at the time of measurement, the “ferrite” referred to in the present invention generally indicates ferrite called bulk ferrite, polygonal ferrite, equiaxial ferrite, etc. and does not include acicular ferrite formed at a lower temperature. However, even with bulk ferrite, depending on the control of the austenite before transformation, sometimes anisotropy occurs in the growth direction and bulk ferrite having a form long in the rolling direction is produced. This is included in the ferrite in the present invention.

Further, to obtain an excellent weldability and toughness, the amounts of addition of the alloy elements have to be adjusted. When X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less, the weld fractures can be greatly reduced. Not only this, the toughness and weld heat affected zone toughness also become excellent. Therefore, X1 is defined as 0.24 or less. Note that when X1 is 0.21 or less, this effect appears more remarkably, so preferably X1 is made 0.21 or less. Note that the C, Mn, Cu, Cr, Si, Ni, Mo, V, and B when calculating the X1 are all amounts of addition expressed by mass %.

Further, the inventors engaged in intensive studies on the method of production of steel plate described in (2) of the present invention having a plate thickness of 4 to 100 mm or so, a matrix tensile strength of 570 to 720 MPa or so, and excellent in all of machineability, matrix toughness, and weldability. As a result, they discovered that by making a composite structure of a soft structure mainly comprised of ferrite and a hard structure mainly comprised of bainite and martensite the main structure of the steel, by strictly defining the balance of the amounts of addition of Si, Cr, Mo, and Mn in the steel ingredients, by strictly defining the method of production mainly comprised of temperature control in a method of production requiring water cooling, etc., the machineability is improved while securing the strength, matrix toughness, and weldability.

Further, the inventors engaged in intensive studies on the method of production of steel plate described in (3) having a plate thickness of 4 to 100 mm or so and a matrix strength of 570 to 720 MPa or so, extremely excellent in machineability, having a toughness of an extent enabling use for welded structures. As a result, they discovered that by making a composite structure of a soft structure mainly comprised of ferrite and a hard structure mainly comprised of bainite and martensite the main structure of the steel, strictly defining the balance of the amounts of addition of Si, Cr, Mo, and Mn among the steel ingredients, increasing the amount of addition of S in a range not greatly reducing the toughness, strictly defining the method of production mainly comprised of temperature control in a method of production requiring water cooling, etc., the machineability is greatly improved while securing the strength and matrix toughness.

Note that the “weldability” referred to in the present invention indicates both weld fractures and weld heat affected zone toughness. The more difficult the occurrence of weld fractures or the higher the weld heat affected zone toughness; the better the weldability. On the other hand, the “machineability” indicates the tool life, machining resistance, and chip control. The longer the tool life, the lower the machining resistance, and the easier the chip control, the better the machineability.

The most important requirements for realizing excellent machineability in the present invention are the following two points in the steel plates described in (2) and (3) of the present invention, but in the steel plate described in (3) of the present invention, there is further the later explained point (third point). First, the first point is to make the structure of the steel plate a composite structure mainly comprised of soft ferrite and hard bainite and martensite. In particular, since steel plate having a plate thickness of 4 to 100 mm or so is covered, it is important that broad locations in the plate thickness direction become a soft and hard composite structure. By controlling the structure in this way, the soft part easily deforms at the time of machining, while stress concentrates near the interface of the soft parts and hard parts and thereby ductile fracture is promoted and as a result machining proceeds with extremely small plastic deformation. Due to this, the tool life becomes longer, the machining resistance falls, and chip control becomes easy. Even in the case of a composite structure mainly comprised of soft ferrite and hard bainite and martensite, if the soft ferrite fraction becomes lower than 30%, the machineability greatly falls, while if over 90%, the strength becomes insufficient, so the ferrite fraction is defined as 30% to 90% and the balance is defined as being mainly comprised of bainite and martensite. Further, when the ferrite fraction is 45% or more, the machineability is further excellent, so preferably the ferrite fraction is defined as 45% to 90% and the balance is defined as being mainly bainite and martensite. Further, when the ferrite fraction is 60% or more, the machineability is remarkably excellent, so more preferably the ferrite fraction is defined as 60% to 90% and the balance is defined as being mainly bainite and martensite. Note that the hard structure is mainly comprised of bainite and martensite, but even when partially including pearlite, acicular ferrite, and other inclusions, in the range defined by the present invention, the machineability does not deteriorate and remains excellent.

The ferrite fraction defined above is measured by observation of the microstructure under an optical microscope. The measurement plane is made the plane formed by the rolling direction and plate thickness direction (hereinafter called the “L plane”). The measurement locations in the plate thickness direction were made, in the case of a plate thickness of 8 mm or less, the three locations of the locations exactly ¼, ½, and ¾ of the plate thickness inside the steel plate from the top surface of the steel plate (hereinafter called the “t/4 part”, “t/2 part”, and “3t/4 part”) and, in the case of a plate thickness of over 8 mm, the three locations of the t/4 part, t/2 part, and 3t/4 part of the plate thickness direction plus the locations exactly 2 mm inside the steel plate from the top surface and rear surface of the steel plate surface (hereinafter called the “top surface 2 mm part” and “rear surface 2 mm part”). The line segments connecting the measurement points are designed to be parallel to the plate thickness direction. The measurement locations in the width direction avoid the ends of the steel plate in the width direction and the spans of locations inside the ends in the width directions by exactly the lengths corresponding to the plate thickness. The measurements are made at locations close to the center part in the width direction as much as possible. The measurements are preferably performed at magnifications of 100× to 500×. An eyepiece with a lattice is used for measurement by the point count method. The average value of the ferrite fractions at all of the measurement locations is used as the ferrite fraction in the present invention. Note that regarding the criteria for judgment of ferrite at the time of measurement, the “ferrite” referred to in the present invention generally indicates ferrite called bulk ferrite, polygonal ferrite, equiaxial ferrite, etc. and does not include acicular ferrite formed at a lower temperature. However, even with bulk ferrite, depending on the control of the austenite before transformation, sometimes anisotropy occurs in the growth direction and bulk ferrite having a form long in the rolling direction is produced. This is included in the ferrite in the present invention.

Next, the second point of the requirements for realizing excellent machineability is as follows: A composite structure of a soft structure mainly comprised of ferrite and a hard structure mainly comprised of bainite and martensite is, as explained above, excellent in machineability, but with this alone, the machineability is not necessarily sufficient for the drilling or surface machining etc. in fabrication of welded structures. It is necessary to optimize the ratio of the amounts of addition of specific alloy elements assuming a composite structure of a soft structure and a hard structure. Specifically, the rates of addition of the amount of Mn, the amount of Si, the amount of Cr, and the amount of Mo are strictly defined. Boring, surface machining, and other machining are in a sense fracture phenomenon of cut materials by tools at a high temperature and high strain rate. How much the energy required for this can be reduced is important, so it is necessary to make the difference in strength between the soft part and hard part at a high temperature large. If the amount of addition of Mn is large, the amount of solution strengthening of the soft ferrite becomes large and the difference in strength of the hard part and soft part is reduced, so the amount of addition of Mn is preferably low. On the other hand, increases in the amounts of addition of Si, Cr, and Mo contribute to the increase in the ordinary temperature strength of the hard part mainly comprised of bainite and martensite and raise the resistance of the hard part to a drop in strength at a high temperature, so have the effect of increasing the difference in strength between the soft part and hard part. The inventors used steel ingots of ingredients with amounts of addition of Mn, Si, Cr, and Mo changed in various ways to produce steels of composite structures of soft structures and hard structures and studied their machineability and ingredient balance and as a result discovered that if the X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is less than 0.15, the absolute level of the machineability is insufficient while conversely if the X2 is over 10.0, the weldability greatly falls. Accordingly, in the present invention, the X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is defined as 0.15 to 10.0. Note that when the value of X2 is 0.3 or more, the machineability is further improved, so preferably X2 is made 0.3 to 10.0. Further, when the value of X2 is 0.4 or more, the machineability is improved more remarkably, so more preferably X2 is made 0.4 to 10.0. Note that the Si, Mo, Cr, and Mn when calculating X2 are all amounts of addition expressed by mass %. In the present invention, Cr and Mo are important elements, but are added as need after consideration of the alloy costs. When Cr and Mo are not added, the value of said X2 is calculated from the amounts of Si and Mn.

In addition to the above most important two requirements for achieving excellent machineability in the steel plate described in (3) of the present invention as explained above, the third point in the requirements for achieving further improved machineability is that it is important to add as large an amount of S as possible to a range riot causing the weldability and toughness to greatly fall. S has the function of reducing the machining resistance and extending the tool life by the effect of MnS as a source of stress concentration. If the amount of addition is over 0.01%, the improvement in the machineability becomes remarkable, while if over 0.035%, the toughness and weldability both fall, so the amount of S is defined as over 0.01% to 0.035%.

The above summarizes the most important requirements for achieving excellent machineability in steel plates described in (2) and (3) of the present invention, but regarding the first point, that is, when the structure is complicated or an extremely fine grain structure, it is sometimes difficult to define a composite structure of a soft structure and a hard structure from observation under an optical microscope. In the present invention, instead, the method of judgment of a composite structure by the micro Vickers hardness is defined in combination. The micro Vickers hardness requires a smaller measurement area than the Vickers hardness, so in the case of a composite structure, the measurement value greatly fluctuates depending on the structure. In particular, a region mostly comprised of ferrite becomes low in hardness. It is possible to use the ratio of number of measurement points with a low hardness to define a composite structure of a soft structure and a hard structure. The inventors ran various structures through micro Vickers hardness tests and clarified the range of micro Vickers hardness giving an excellent machineability. As a result, when the ratio of micro Vickers hardness of 190 HV or less is 20% or more, the machineability is excellent, so the ratio of micro Vickers hardness of 190 HV or less is made 20% or more. Further, when the ratio of micro Vickers hardness of 180 HV or less is 20% or more, the machineability is excellent, so preferably the ratio of micro Vickers hardness of 180 HV or less is made 20% or more. Further, when the ratio of micro Vickers hardness of 170 HV or less is 20% or more, the machineability is further excellent, so more preferably the ratio of micro Vickers hardness of 170 HV or less is made 20% or more. Note that when the ratio of micro Vickers hardness of 170 HV or less is 40% or more, the machineability is further improved, so more preferably the ratio of micro Vickers hardness of 170 HV or less is made 40% or more.

The “micro Vickers hardness” referred to in the present invention is the value measured by the method defined in JIS Z 2244. Another method of measurement besides that defined in the standards will be explained in detail here. The test force is made 0.09807N. The measurement plane is made the L plane. The measurement locations in the plate thickness direction are made, in the case of a plate thickness of 8 mm or less, the three locations of the t/4 part, t/2 part, and 3t/4 part, and, in the case of a plate thickness of over 8 mm, five locations including also the top surface 2 mm part and rear surface 2 mm part. The line segments connecting the measurement points are designed to become parallel to the plate thickness direction. The measurement locations in the width direction avoid the ends of the steel plate in the width direction and the spans of locations inside the ends in the width directions by exactly the lengths corresponding to the plate thickness. The measurements are made at locations close to the center part in the width direction as much as possible. The measurements are conducted at intervals of 100 μm as shown in FIG. 1. The number of measurement points is made 121 points. The ratio of the number of points among the 121 points having a micro Vickers hardness of 190 HV or less is measured. When the plate thickness is 8 mm or less, the average value of three locations is calculated, while when the plate thickness is over 8 mm, the average value of five points is calculated. This value is used as the ratio of micro Vickers hardness of 190 HV or less. The ratio of micro Vickers hardness of 180 HV or less and the ratio of 170 HV or less are measured by the same technique.

The above definition is important for improving the machineability, but to secure the strength, toughness, and weldability, the following definitions become necessary.

First, to secure a tensile strength of 570 MPa or more, the Vickers hardness has to be defined. If the Vickers hardness drops below 165 HV, securing a tensile strength of 570 MPa or more becomes difficult, while if over 300 HV, the weldability greatly falls, so the Vickers hardness is defined as 165 HV to 300 HV.

The Vickers hardness referred to in the present invention is the value measured by a method defined in JIS Z 2244. Another method of measurement besides that defined in the standards will be explained in detail here. The test force is made 98.07N. The measurement plane is made the L plane. The measurement locations in the plate thickness direction are made, in the case of a plate thickness of 8 mm or less, the three locations of the t/4 part, t/2 part, and 3t/4 part, and, in the case of a plate thickness of over 8 mm, five locations including also the top surface 2 mm part and rear surface 2 mm part. The line segments connecting the measurement points are designed to become parallel to the plate thickness direction. The measurement locations in the width direction avoid the ends of the steel plate in the width direction and the spans of locations inside the ends in the width directions by exactly the lengths corresponding to the plate thickness. The measurements are made at locations close to the center part in the width direction as much as possible. The measurements are conducted at five points or more for each location and the average value of the locations is calculated. In the case of a plate thickness of 8 mm or less, the average value of three locations is calculated, while in the case of a plate thickness of over 8 mm, the average value of five points is calculated. This is used as the Vickers hardness.

Further, to obtain an excellent weldability and toughness, the amounts of addition of the alloy elements have to be adjusted. When the X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less, not only are weld fractures greatly reduced, but also the toughness and weld heat affected zone toughness become excellent, so X1 is defined as 0.24 or less. Note that when X1 is 0.21 or less, this effect appears more remarkably, so preferably X1 is made 0.21 or less. Note that the C, Mn, Cu, Cr, Si, Ni, Mo, V, and B when calculating X1 are all amounts of addition expressed by mass %.

Below, the ranges of the alloy elements of the steel plate of the present invention will be defined.

The following, unless particularly indicated to the contrary, is common for the steel plates described in (1), (2), and (3) of the present invention.

C is an element required for securing strength, so the amount of addition is made 0.005% or more. However, on the other hand, an increase in the amount of C invites a drop in matrix toughness and a drop in weldability due to the formation of coarse precipitates, so the upper limit is made 0.2%. Note that if the amount of C is 0.07% or more, securing a tensile strength of 570 MPa or more becomes easy, while if 0.14% or less, the toughness and weldability become more excellent, so preferably the amount of C is made 0.07% to 0.14%.

Si is an extremely important element in the present invention. It is an element effective for increasing the strength and at the same time improving the machineability and for causing the formation of ferrite in a broad range of plate thickness in a method of production predicated on water cooling after rolling and then transforming the balance to mainly bainite and martensite to obtain a composite structure mainly comprised of soft ferrite and hard bainite and martensite. To achieve this effect, addition of 0.01% or more is necessary. Addition of over 1% causes the weldability to drop, so the amount of addition is made 0.01% to 1%. Note that to achieve the above effect more remarkably, addition of 0.2% or more is effective. On the other hand, with 0.55% or less, the weldability becomes extremely excellent, so preferably the amount is made 0.2% to 0.55%.

Mn is an element effective for increasing strength. To achieve the tensile strength of 570 MPa or more covered by the present invention, even at a minimum, 0.01% or more must be added, but conversely if adding over 2%, the weldability falls. Therefore, the amount is defined as 0.01% to 2%. Further, if adding Mn in an amount over 1.4%, the machineability falls, so from the viewpoint of the machineability, 1.4% or less is preferable. Therefore, in the steel plate described in (2) and (3) of the present invention, the amount is defined as 0.01% to 1.4%.

P is an impurity element and preferably has a low amount of addition. Addition over 0.02% causes a drop in the matrix ductility, toughness, and weldability, so the amount is made 0.02% or less.

S is an important element in the present invention and is positively added to improve machineability.

Addition of S causes formation of MnS which acts as a local source of stress concentration so additionally improves the machineability. This effect becomes greater the greater the amount of addition of S, but addition over 0.035% causes the matrix toughness to drop sharply, so the upper limit is defined as 0.035%.

Note that when reducing the amount of addition of S, the effect of improvement of machineability by S becomes smaller, but the matrix toughness and weldability are improved. Therefore, the amount of addition of S is preferably made large when stressing machineability and made small when conversely stressing matrix toughness and weldability.

That is, in the steel plate described in (2) of the present invention, the addition of S has no bearing in the mechanism of improvement of machineability, so a lower amount of addition is preferable. Addition over 0.01% causes the matrix toughness to drop due to the formation of MnS, so 0.01% or less is defined. Note that if the amount of S is 0.006% or less, the matrix toughness is more improved, so preferably the amount of S is defined as 0.006% or less.

Further, in the steel plate described in (3) of the present invention, addition of S functions to reduce the machining resistance and extend the tool life due to the effect of MnS as a source of stress concentration. If the amount of addition is over 0.01%, the improvement in the machineability becomes remarkable, while if over 0.035%, the toughness and weldability both fall, so the amount of S is made over 0.01% to 0.035%.

Al is an element effective as a deoxidizing material. The amount of addition is made 0.001% or more. However, on the other hand, an increase of the amount of Al invites a drop in the matrix toughness, so the upper limit is made 0.1%.

N is an impurity element. Addition over 0.01% causes the matrix toughness and weldability to drop, so 0.01% or less is defined.

Mo is an extremely important element in the present invention and can be added as needed after considering the cost. It is an element effective for increasing the strength and at the same time improving the machineability and for causing the formation of ferrite in a broad range of plate thickness in a method of production predicated on water cooling after rolling and then transforming the balance to mainly bainite and martensite to obtain a composite structure mainly comprised of soft ferrite and hard bainite and martensite. To achieve this effect, addition of 0.01% or more is necessary. Addition of over 1% causes the weldability to drop, so the amount of addition is made 0.01% to 1%. Note that to achieve the above effect more remarkably, addition of 0.1% or more is effective, so preferably the amount is made 0.1% to 1.0%.

Cr is an extremely important element in the present invention and may be added as needed after consideration of the cost. It is an element effective for increasing the strength and at the same time improving the machineability and for causing the formation of ferrite in a broad range of plate thickness in a method of production predicated on water cooling after rolling and then changing the balance to mainly bainite and martensite to obtain a composite structure mainly comprised of soft ferrite and hard bainite and martensite. To achieve this effect, addition of 0.01% or more is necessary. Addition of over 1% causes the weldability to drop, so the amount of addition is made 0.01% to 1%. Note that to achieve the above effect more remarkably, addition of 0.1% or more is effective, so preferably the amount is made 0.1% to 1.0%.

In the present invention, Nb, Ti, and V are also important elements. Nb, Ti, and V are elements effective for increasing the strength by precipitation strengthening etc. and for improving the toughness by making the structure finer and are added as needed from the viewpoint of securing the strength and toughness. The inventors evaluated the tool life when drilling steel plate comprised of a soft and hard composite structure strengthened by these elements. As a result, they discovered that even with a soft and hard composite structure, if the amount of precipitation strengthening is large, the difference in hardness of the soft part and hard part is reduced and the drill life falls. If any of the amounts of addition of Nb, Ti, and V exceeds 0.1%, the machineability remarkably falls. On the other hand, with addition of less than 0.001%, the effect of increase of strength is not obtained. Therefore, the amounts of addition of Nb, Ti, and V are made 0.001% to 0.1%. Note that when the amounts of addition of Nb, Ti, and V are 0.05% or less, 0.04% or less, and 0.05% or less, the drop in machineability accompanying the increase in strength is particularly small, so preferably the amounts of addition of Nb, Ti, and V are made 0.05% or less, 0.04% or less, and 0.05% or less.

Cu, Ni, and B are added as needed from the viewpoint of securing the strength. Cu is an element effective for securing strength. With addition of less than 0.005%, the effect is small, while addition of over 1% causes the weldability to drop, so the range is made 0.005 to 1%. Ni is added as needed to secure strength. With addition of less than 0.01%, the effect is small, while addition of over 2% causes the weldability to drop, to the range is made 0.01 to 2%. B is an element effective for increasing hardenability. The amount of addition is made 0.0002% or more. However, on the other hand, an increase of the amount of B invites a drop in the matrix toughness due to formation of coarse precipitates, so the upper limit is made 0.005%.

Addition of one or more of a REM, Ca, Zr, and Mg can improve the matrix toughness and weld heat affected zone toughness through control of matrix inclusions, increased fineness of the heated austenite of the weld heat affected zone, and formation of transformation nuclei from inside the grains, so these are added as necessary. To achieve this effect, addition of 0.0005% or more of any or REM, Ca, Zr, or Mg is necessary. On the other hand, if overly added, the sulfides and oxides become coarser and the matrix toughness and ductility are lowered, so the upper limit value is made 0.1% for REM and 0.02% for Ca, Zr, and Mg.

Note that in melting the steel of the present invention, the present invention is not impaired in effect in any way even if the O, Zn, Sn, Sb, Te, Ta, W, Pb, Bi, etc. which may dissolve out from the materials used including the added alloys or the furnace materials during melting and enter the steel as unavoidable impurities is 0.005% or less.

Next, the methods of production of steel plate described in (1) of the present invention will be explained. Roughly classified, there are three methods of production. The first method of production (method of production 1) is a method performing rolling at a relatively low temperature, then speedily water cooling, the second method of production (method of production 2) is the method of air cooling until the formation of ferrite after rolling and then continuing with water cooling, and the third method of production (method of production 3) is the method of heating again when the temperature of the rolled steel plate falls, then water cooling. In each case, to form ferrite in a broad range in the plate thickness direction and secure a high ferrite fraction near the steel plate surface, it is necessary to strictly control the temperature in accordance with the plate thickness.

Note that when producing the steel plate described in (1), a steel slab or cast slab having the steel composition described in (1) of the present invention, that is, C, 0.005 to 0.2%, Si: 0.01 to 1%, Mn: 0.01 to 2%, P: 0.02% or less, S: 0.035% or less, Al: 0.001 to 0.1%, N, 0.01% or less, and the balance of iron and unavoidable impurities, where X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less, can be used.

First, the first method of production (method of production 1), that is, the method of rolling at a relatively low temperature, then speedily water cooling and the method of production described in (5) of the present invention will be explained. In this method of production, strict definition of the rough rolling, finish rolling, and water cooling are important.

The rough rolling is important for making the austenite finer by recrystallization and stably forming ferrite and as a result improving the machineability. If the total reduction rate of the rough rolling becomes less than 30%, the ferrite is not stably formed, while if over 95%, the productivity greatly falls, so the total reduction rate of the rough rolling is defined as 30% to 95%. Note that when the total reduction rate of the rough rolling is 50% or more, the machineability is improved more, so preferably the total reduction rate of the rough rolling is made 50% to 95%. Further, when the total reduction rate of the rough rolling is 80% or more, the machineability is improved still more, so more preferably the total reduction rate of the rough rolling is made 80% to 95%. The temperature for the rough rolling may be freely set so long as the condition of the finish rolling temperature is satisfied. Note that the “total reduction rate of the rough rolling” is the plate thickness before rough rolling minus the plate thickness after rough rolling divided by the plate thickness before rough rolling expressed as a percentage.

The finish rolling is important for actively utilizing the dislocations introduced in the unrecrystallization temperature range in various manners so as to promote the formation of ferrite and make it finer and as a result improve the machineability, toughness, and weldability. The inventors produced steel plates of various alloy ingredients and plate thicknesses by this method of production and evaluated them for machineability, weldability, and matrix toughness. As a result, they confirmed that when making the first pass bite temperature of the finish rolling a temperature of not more than T1 (° C.) expressed by T1=35 ln(X2/2)−25√t+1070, X2=(Si/5+Mo+Cr/2)/Mn, ferrite is formed in a broad plate thickness direction and the machineability, weldability, and toughness are all excellent. Accordingly, the first pass bite temperature of the finish rolling is defined as not more than T1 (° C.) expressed by T1=35 ln(X2/2)−25√t+1070, X2=(Si/5+Mo+Cr/2)/Mn. Note that Si, Mo, Cr, and Mn indicate amounts of addition expressed by mass %, while t is the thickness (mm) of the steel plate. If the first pass bite temperature of the finish rolling is less than 720° C., working of the ferrite will cause the matrix toughness and machineability to greatly fall, so the first pass bite temperature of the rolling is given a lower limit of 720° C. Note that if making the first pass bite temperature of the finish rolling 40° C. lower than T1, the machineability is improved more remarkably, so preferably the first pass bite temperature of the finish rolling is made a temperature 40° C. lower than T1. Further, if making the first pass bite temperature of the finish rolling 80° C. lower than T1, the machineability is improved still more remarkably, so more preferably the first pass bite temperature of the finish rolling is made a temperature 80° C. lower than T1. If the final pass bite temperature of the finish rolling is less than 700° C., working of the ferrite will cause the matrix toughness and machineability to greatly fall, while if over T1 (° C.), the ferrite will not be produced in a broad range in the plate thickness direction, so the final pass bite temperature of the finish rolling preferably has a lower limit of 700° C. and an upper limit of T1 (° C.). The total reduction rate is also important for the finish rolling. If less than 30%, ferrite is not formed over a broad range of the plate thickness, while conversely if over 95%, the productivity greatly falls, so the total reduction rate of the finish rolling is defined as 30% to 95%. Further, if the total reduction rate of the finish rolling is 60% or more, the ferrite is formed more stably and the machineability is improved, so preferably the total reduction rate of the finish rolling is made 60% to 95% or less.

Note that in the present invention, the rolling performed by a rough rolling machine is deemed rough rolling, while the rolling performed by a finish rolling machine is deemed finish rolling. If performing rough rolling and finish rolling by the same rolling machine, when there is a clear temperature setting dividing the rolling into a first half and a second half, the first half of rolling is deemed the rough rolling and the second half of rolling is deemed the finish rolling. When there is no clear temperature setting or when there are two or more temperature settings, all of the rolling passes after and including the rolling pass where the temperature of the steel plate surface before the start of that rolling pass became 950° C. or less are deemed the finish rolling. The “first pass bite temperature of the finish rolling” indicates the temperature measured at the surface of the steel plate before the first reduction by the finish rolling. The final pass bite temperature of the finish rolling indicates the temperature measured at the surface of the steel plate before the final reduction by the finish rolling. Note that steel plate surface temperature can be measured for example by using a radiant thermometer.

Next, the water cooling will be explained. Water cooling is effective for securing 570 to 720 MPa or so of tensile strength, securing strength with low alloy content and thereby improving the weldability, and further making the structure finer and thereby improving the matrix toughness. If the flow rate at the time of water cooling falls below 0.2 m³/m²·min, the strength falls, while if over 5.0 m³/m²·min, the ferrite is no longer stably formed in a broad range in the plate thickness direction and the machineability falls, so the flow rate at the time of water cooling is defined as 0.2 m³/m²·min to 5.0 m³/m²·min. If the end temperature of the water cooling exceeds 600° C., the residual austenite after ferrite formation will not transform at a low temperature and the strength will fall, so the end temperature of the water cooling is made 600° C. or less. Here, the “end temperature of the water cooling” means the maximum value of the steel plate surface temperature measured after waiting for heat recovery after water cooling. After water cooling, air cooling is performed.

In the water cooling performed after the end of rolling, by changing the first half and second half cooling rates, it is possible to form the ferrite more stably, so this technique may be adopted if necessary. By making the cooling rate of the first half defined by the water cooling start temperature to over 650° C. 1° C./s to 5° C./s and making the second half cooling rate of the second half defined by 650° C. to the water cooling end temperature 10° C./s to 100° C./s, steel plate with an even more excellent machineability and an equal or better strength can be produced. The cooling rate in the first half of the cooling is lowered so as to increase the amount of production of ferrite and make the C in the untransformed austenite more concentrated and thereby lower the transformation temperature of the bainite etc. formed in the second half cooling, while the cooling rate in the latter half is raised so as to make the transformation temperature of the untransformed austenite as low as possible. Note that the temperatures and cooling rates in this two-stage cooling are made the temperatures measured at the steel plate t/4 part and the average cooling rates calculated based on those values and can be measured using a spare sample of a steel plate in which a thermocouple is embedded and performing water cooling simulating actual water cooling.

Below, other preferable conditions in the first method of production (method of production 1) will be explained. Before the rough rolling and finish rolling, the steel slab or cast slab is heated. If the heating temperature is less than 900° C., part of the structure from before the heating will remain untransformed, so the material will become nonhomogeneous, while if the heating temperature exceeds 1350° C., the austenite will become coarser, the final structure will also become coarser, the matrix toughness will greatly fall, and also the formation of ferrite will be suppressed and the machineability will fall, so the heating temperature is preferably made 900° C. to 1350° C. The water cooling is to be started very speedily after the end of the finish rolling. For example, it is preferably started within 200 from the end of the finish rolling. This is because if the time until the start of the water cooling exceeds 200 s, the dislocations introduced by the rolling will be reduced by recovery, ferrite will not be stably formed in a broad range in the plate thickness direction, and the machineability will fall. Here, the “end of the finish rolling” means the point of time when the frontmost part of the steel plate is reduced at the final pass of the finish rolling, while the “start of the water cooling” means the point of time when the frontmost part of the steel plate reaches the water cooling facility and the water cooling starts. Further, the water cooled, then air cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.

Next, the second method of production (method of production 2), that is, the method of air cooling until formation of ferrite after rolling, then water cooling, and the method of production described in (7) of the present invention will be defined. The heating is similar to the first method of production. The temperature of the rough rolling can be freely set, but if the total reduction rate of the rough rolling is less than 30%, the toughness greatly falls, while if over 95%, the productivity greatly falls, so the total reduction rate of the rough rolling is defined as 30% to 95%. The temperature of the finish rolling is not defined like in the first method of production and can be any condition. If the total reduction rate of the finish rolling is less than 30%, the toughness greatly falls, while if over 95%, the productivity greatly falls, so the total reduction rate of the finish rolling is defined as 30% to 95%. After the heating, rough rolling, and finish rolling end, air cooling is performed. During the air cooling, ferrite is formed, then water cooling is performed. The inventors investigated steels of various ingredients by changing the steel plate surface temperature when shifting from air cooling after finish rolling to water cooling in various ways and discovered that when the steel plate surface temperature when shifting to water cooling is T2 (° C.) expressed by T2=910−310×C−80×Mn−20×Cu−15×Cr−55×Ni−80×Mo+0.0006t²−0.56t−8.6 or less, ferrite is formed in a broad range in the plate thickness direction and the machineability is improved, while when the steel plate surface temperature falls below 650° C., the strength greatly falls. Accordingly, the steel plate surface temperature when shifting to water cooling is defined as T2 (° C.) expressed by T2=910−310×C−80×Mn−20×Cu−15×Cr−55×N−80×Mo+0.0006t²−0.56t−8.6 to 650° C. Here, the “steel plate surface temperature when shifting to water cooling” means the steel plate surface temperature measured before water cooling. C, Mn, Cu, Cr, Ni, and Mo indicate the amounts of addition of the corresponding elements (mass %), while t indicates the plate thickness (mm). If the flow rate at the time of water cooling falls below 0.2 m³/m²·min, the strength falls, while if over 5.0 m³/m²·min, the ferrite is no longer stably formed in a broad range in the plate thickness direction and the machineability falls, so the flow rate at the time of water cooling is defined as 0.2 m³/m²·min to 5.0 m³/m²·min. If the end temperature of the water cooling exceeds 500° C., the residual austenite after formation of ferrite will not transform at a low temperature and the strength will falls, so the end temperature of the water cooling is made 500° C. or less. Here, the “end temperature of water cooling” means the maximum value of the steel plate surface temperature measured after waiting for heat recovery after water cooling. After water cooling, air cooling is performed. Further, the water cooled, then air cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.

Next, the third method of production (method of production 3), that is, the method of heating again after the temperature of the steel plate falls after rolling will be defined. The heating before the rolling is the same as the first method of production. The temperature of the rough rolling may be freely set, but if the total reduction rate of the finish rolling is less than 30%, the toughness will greatly fall, while if it exceeds 95%, the productivity greatly falls. Accordingly, the total reduction rate of the finish rolling is defined as 30% to 95%. The temperature of the finish rolling is not defined as in the first method of production and can be made any condition. If the total reduction rate of the finish rolling is less than 30%, the toughness will greatly fall, while if over 95%, the productivity greatly falls, so the total reduction rate of the finish rolling is defined as 30% to 95%. After the heating, rough rolling, and finish rolling, the steel plate is cooled to 500° C. or less by any means, then is again heated. The inventors investigated the reheating temperature by changing it in various ways. If the reheating temperature is less than T3 (° C.) expressed by T3=0.0017t²+0.17t+730 or if it is over 850° C., sufficient strength cannot be obtained, so the reheating temperature is defined as T3 (° C.) expressed by T3=0.0017t²+0.17t+730 to 850° C. After reheating, water cooling is performed. If the flow rate at the time of water cooling falls below 0.2 m³/m²·min, the strength falls, while if over 5.0 m³/m²·min, the ferrite is no longer stably formed in a broad range and the machineability falls, so the flow rate at the time of water cooling is defined as 0.2 m³/m²·min to 5.0 m³/m²·min. If the end temperature of the water cooling exceeds 500° C., the residual austenite after ferrite formation will not transform at a low temperature and the strength will fall, so the end temperature of the water cooling is made 500° C. or less. Here, the “end temperature of the water cooling” means the maximum value of the steel plate surface temperature measured after waiting for heat recovery after water cooling. After water cooling, air cooling is performed.

Next, the methods of production of steel plate of (2) and (3) of the present invention will be explained. Roughly classified, there are four methods of production. Methods of production 4 to 7 and 4′ to 7′ will be described for the steel plates. Further, the methods of production of steel plate described in (2) and (3) of the present invention are the same in all four methods of production in all conditions other than the steel compositions of the steel slabs or cast slabs, so in the following explanations, the explanations will be given for the steel plates described in (2) and (3). The first method of production (methods of production 4, 4′) is the method of water cooling speedily after rolling, the second method of production (methods of production 5, 5′) is the method of heating again after the temperature of the steel plate falls after rolling, then water cooling, the third method of production (methods of production 6, 6′) is the method of air cooling until the formation of ferrite after rolling, then water cooling, and the fourth method of production (methods of production 7, 7′) is the method of heating again to the two-phase region after the temperature of the steel plate falls after rolling, then water cooling.

Note that when producing the steel plate described in (2) of the present invention, a steel slab or cast slab having the steel composition described in (2) of the present invention, that is, having the steel composition described in (1) wherein Mn: 0.1% to 1.4%, S: 0.01% or less, having X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, and having X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0 is used, further, when producing the steel plate described in (3) of the present invention, a steel slab or cast slab having the steel composition described in (3) of the present invention, that is, having the steel composition described in (1) wherein Mn: 0.1% to 1.4%, S: over 0.01% to 0.035%, having X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, and having X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0 is used,

First, the first method of production (methods of production 4, 4′) among the methods of production of steel plate described in (2) and (3), that is, the method of starting water cooling speedily after rolling, and the methods of production described in (9) and (14) of the present invention will be explained. In this method of production, the rough rolling, finish rolling, and water cooling become important.

First, the rough rolling will be explained. Rough rolling is important from the viewpoint of increasing the fineness of the austenite through recrystallization and thereby promoting the formation of ferrite. If the total reduction rate of the rough rolling is less than 30%, the ferrite is not stably formed, while if over 95%, the productivity greatly falls, so the total reduction rate of the rough rolling is defined as 30% to 95%. Further, if making the total reduction rate of the rough rolling 50% or more, the ferrite is formed more stably and the machineability is further improved, so preferably the reduction rate in the rough rolling is made 50% to 95% Further, if making the total reduction rate of the rough rolling 80% or more, the ferrite is still more stably formed and the machineability still more improved, so more preferably the reduction rate of the rough rolling is made 80% to 95%. The rough rolling bite temperature and the steel plate surface temperature before the final pass may be freely set so long as the condition of the finish rolling temperature is satisfied. Note that the “total reduction rate of the rough rolling” is the plate thickness before rough rolling minus the plate thickness after rough rolling divided by the plate thickness before rough rolling expressed as a percentage.

The finish rolling is important for stably forming ferrite in the method of production using water cooling. The lower the temperature of the rolling, the higher the density of dislocations introduced per unit reduction rate and the more recovery of dislocations in the middle of transport between rolling passes or from the rolling machine to the water cooling facility is suppressed, so formation of ferrite can be promoted. The behavior of formation of ferrite is heavily influenced by the alloy ingredients, so the finish rolling temperature has to be defined in relation to the ingredients. In general, in thick gauge steel plate having a tensile strength of 570 MPa or more, with the method of production of water cooling speedily after rolling, formation of ferrite is difficult. To achieve this, an extremely low finish rolling temperature becomes necessary and the productivity falls, but in the present invention, it is newly discovered that production is possible without a drop in productivity in a range of ratios of ingredients of Si, Mn, Mo, and Cr defined for improving the machineability. The inventors studied the optimal first pass bite temperature of the finish rolling for steels of various ingredients and discovered that when the first pass bite temperature of the finish rolling is T4 (° C.) expressed by T4=35 ln(X2/2)−25√t−1100 or less, ferrite is stably formed. Accordingly, the first pass bite temperature of the finish rolling is defined as T4 (° C.) or less. Here, X2, as already shown, is the value calculated by X2=(Si/5+Mo+Cr/2)/Mn, and t is the plate thickness (mm). The formula of T4 includes the item of plate thickness because the greater the final plate thickness, the more the reduction rate at the rolling generally falls, so the increasing coarseness of the recrystallization grain size and drop in density of remaining dislocations cause the formation of ferrite to be suppressed and thereby low temperature rolling to become necessary. Note that if the first pass bite temperature of the finish rolling is set to 40° C. lower than T4, the machineability is improved more remarkably, so preferably the first-pass bite temperature of the finish rolling is made a temperature 40° C. lower than T4 or less. Further, if making the first pass bite temperature of the finish rolling 80° C. lower than T4, the machineability is improved still more remarkably, so more preferably the first pass bite temperature of the finish rolling is made a temperature 80° C. lower than T4 or less. Note that if the first pass bite temperature of the finish rolling is lower than the Ar₃ point, the increase in hardness due to working of the ferrite will cause the machineability to fall, so the lower limit of the first pass bite temperature of the finish rolling is defined as the Ar₃ point. The final pass bite temperature of the finish rolling is preferably given a lower limit of a temperature 100° C. lower than the Ar₃ point or more and an upper limit of T4+50 (° C.) from the viewpoint of greatly suppressing any drop in machineability accompanying working of the ferrite.

Note that in the present invention, the rolling performed by a rough rolling machine is deemed rough rolling, while the rolling performed by a finish rolling machine is deemed finish rolling. If performing rough rolling and finish rolling by the same rolling machine, when there is a clear temperature setting dividing the rolling into a first half and a second half, the first half of rolling is deemed the rough rolling and the second half of rolling is deemed the finish rolling. When there is no clear temperature setting or when there are two or more temperature settings, all of the rolling passes after and including the rolling pass where the temperature of the steel plate surface before the start of that rolling pass became 950° C. or less are deemed the finish rolling. The “first pass bite temperature of the finish rolling” indicates the temperature measured at the surface of the steel plate before the first reduction by the finish rolling. The “final pass bite temperature of the finish rolling” indicates the temperature measured at the surface of the steel plate before the final reduction by the finish rolling. The Ar₃ point cannot be directly measured, but can be estimated by thermo-mechanical treatment simulating actual production conditions while measuring the expansion curve. Note that steel plate surface temperature can be measured for example by using a radiant thermometer.

The total reduction rate of the finish rolling is important from the viewpoint of the stable formation of ferrite. If the total reduction rate of the finish rolling is 30% or more, the ferrite is stably formed and thereby the machineability is improved. On the other hand, if the total reduction rate of the finish rolling exceeds 95%, the productivity greatly falls. Accordingly, the total reduction rate of the finish rolling is defined as 30% to 95%. Note that making the total reduction rate of the finish rolling 60% or more improves the machineability more, so preferably the total reduction rate of the finish rolling is made 60% to 95% or less. Note that the “total reduction rate of the finish rolling” is the plate thickness before finish rolling minus the plate thickness after finish rolling divided by the plate thickness before finish rolling expressed as a percentage.

Next, the conditions of the water cooling will be explained. Water cooling is important for simultaneously achieving an improvement in the machineability through stable formation of ferrite and formation of a structure with a balance of mostly bainite and martensite by low temperature transformation, an improvement of matrix toughness by increasing the fineness of the grain size, and improvement of the weldability through securing strength with a low alloy content. If the flow rate at the time of water cooling falls below 0.2 m³/m²·min, the strength falls, while if over 5.0 m³/m²·min, the ferrite is no longer stably formed and the machineability falls, so the flow rate at the time of water cooling is defined as 0.2 m³/m²·min to 5.0 m³/m²·min. If the end temperature of the water cooling exceeds 600° C., the residual austenite after ferrite formation will not transform at a low temperature and the strength will fall, so the end temperature of the water cooling is made 600° C. or less. Here, the “end temperature of the water cooling” means the maximum value of the steel plate surface temperature measured after waiting for heat recovery after water cooling. After water cooling, air cooling is performed.

Further, the water cooling is preferably started speedily after the end of the finish rolling. This is because if the time from the end of the finish rolling to the start of the water cooling becomes long, the dislocations introduced by the rolling will be reduced by recovery, ferrite will not be stably formed, and the machineability will fall. Note that specifically the water cooling is preferably started within 200 s from the end of the finish rolling. Here, the “end of the finish rolling” means the point of time when the frontmost part of the steel plate is reduced at the final pass of the finish rolling, while the “start of the water cooling” means the point of time when the frontmost part of the steel plate reaches the water cooling facility and the water cooling starts.

In the water cooling performed after the end of rolling, by changing the first half and second half cooling rates, it is possible to form the ferrite more stably, so this technique may be adopted if necessary. By making the cooling rate of the first half defined by the water cooling start temperature to over 650° C. 1° C./s to 5° C./s and making the second half cooling rate of the second half defined by 650° C. to the water cooling end temperature 10° C./s to 100° C./s, steel plate with an even further improved machineability and an equal or better strength can be produced. The cooling rate in the first half of the cooling is lowered so as to increase the amount of production of ferrite and make the C in the untransformed austenite more concentrated and thereby lower the transformation temperature of the bainite etc. formed in the second half cooling, while the cooling rate in the latter half is raised so as to make the transformation temperature of the untransformed austenite as low as possible. Note that the temperatures and cooling rates in this two-stage cooling are made the temperatures measured at the steel plate t/4 part and the average cooling rates calculated based on those values and can be measured using a spare sample of a steel plate in which a thermocouple is embedded and performing water cooling simulating actual water cooling.

Below, other preferable conditions in the method of production will be explained. Before the rolling, the steel slab or cast slab is heated. If the heating temperature is less than 900° C., part of the structure from before the heating will remain untransformed, so the material will become nonhomogeneous, while if the heating temperature exceeds 1350° C., the austenite will become coarser, the final structure will also become coarser, the matrix toughness will greatly fall, and also the formation of ferrite will be suppressed and the machineability will fall, so the heating temperature is preferably made 900° C. to 1350° C. Further, the water cooled, then air cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.

Next, the second method of production (methods of production 5, 5′) among the methods of production of steel plate described in (2) and (3) of the present invention, that is, the method of heating again when the temperature of the steel plate falls after rolling, then water cooling, and the methods of production described in (11) and (16) of the present invention will be defined. The heating, rough rolling, and finish rolling may be performed under any conditions. After the end of the heating, rough rolling, and finish rolling, the steel plate is cooled to 500° C. or less by any means, then is again heated to 900° C. to 1050° C. After the heating, it is water cooled by a cooling rate of 1° C./s to 100° C./s. The end temperature of the water cooling is made 500° C. or less. After the water cooling, air cooling is performed.

Due to the reheating after rolling, fine austenite is obtained and ferrite can be stably formed. If the reheating temperature is less than 900° C., austenite is formed with a high concentration of C. This becomes martensite after transformation and causes the matrix toughness to greatly fall. Further, if the reheating temperature is over 1050° C., ferrite is not stably produced, and the machineability falls. Therefore, the reheating temperature is defined as 900° C. to 1050° C. If the cooling rate after reheating is less than 1° C./s, the residual austenite after formation of ferrite will not transform at a low temperature and the strength will fall, while if the cooling rate exceeds 100° C./s, ferrite will not be stably formed, so the cooling rate after reheating is defined as 1° C./s to 100° C./s. If the end temperature of the water cooling exceeds 500° C., the residual austenite after formation of ferrite will not transform at a low temperature and the strength will fall, so the end temperature of the water cooling is defined as 500° C. or less. Further, the water cooled, then air cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.

Here, the “end temperature of water cooling” means the maximum value of the steel plate surface temperature measured speedily after heat recovery after water cooling. The cooling rate of the water cooling is made the average cooling rate calculated based on the temperatures measured at the steel plate t/4 part and can be estimated by using a spare sample of a steel plate in which a thermocouple is embedded and performing water cooling simulating actual water cooling.

Next, the third method of production (methods of production 6, 6′) among the methods of production of steel plate described in (2) and (3) of the present invention, that is, the method of air cooling until the start of formation of ferrite after rolling, then water cooling, and the methods of production described in (12) and (17) of the present invention will be defined. The heating is the same as said first method of production (method of production 4). In the rough rolling, if the total reduction rate is less than 30%, the toughness falls, while if over 95%, the productivity greatly falls, so the total reduction rate of the rough rolling is defined as 30% to 95%. The finish rolling is not defined as to temperature like with the first method and may be performed under any conditions. If the total reduction rate of the finish rolling is less than 30%, the toughness falls, while if over 95%, the productivity greatly falls, so the total reduction rate of the finish rolling is defined as 30% to 95%. After the heating, rough rolling, and finish rolling end, air cooling is performed. During the air cooling, ferrite starts to form, then water cooling is performed. When the temperature for starting the water cooling exceeds the Ar₃ point, the ferrite is not stably formed and the machineability falls, while if lower than a temperature lower than the Ar₃ point by 150° C., the strength falls, so the water cooling start temperature is defined as the Ar₃ point to a temperature lower than the Ar₃ point by 150° C. or higher. Here, the “water cooling start temperature” means the steel plate surface temperature measured before water cooling. The Ar₃ point can be estimated by thermo-mechanical treatment simulating actual production conditions while measuring the expansion curve. If the flow rate at the time of water cooling falls below 0.2 m³/m²·min, the strength will fall, while if over 5.0 m³/m²·min, the productivity will fall, so the flow rate at the time of water cooling is defined as 0.2 m³/m²·min to 5.0 m³/m²·min. If the end temperature of the water cooling exceeds 500° C., the residual austenite after formation of ferrite will not transform at a low temperature and the strength will fall, so the end temperature of the water cooling is made 500° C. or less. Here, the “end temperature of water cooling” means the maximum value of the steel plate surface temperature measured speedily after heat recovery after water cooling. After the water cooling, air cooling is performed. Further, the water cooled, then air cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.

Next, the fourth method of production (methods of production 7, 7′) among the methods of production of steel plate described in (2) and (3) of the present invention, that is, the method of heating again to the two-phase region when the temperature of the steel plate falls after rolling, and the methods of production described in (13) and (18) of the present invention will be defined. The heating, rough rolling, and finish rolling are similar to said third method of production (methods of production 6, 6′). After the heating, rough rolling, and finish rolling end, the steel plate is cooled to 500° C. or less by any means, then is again heated. If the reheating temperature is less than 730° C., the machineability falls, while if 900° C. or more, the strength falls, so the reheating temperature is defined as 730° C. to less than 900° C. After reheating, the plate may be water cooled by any method. If the end temperature of water cooling is over 500° C., the residual austenite after formation of ferrite will not transform at a low temperature and the strength will fall, so the end temperature of water cooling is made 500° C. or less. After the water cooling, air cooling is performed. Further, the cooled steel plate may be heat treated if necessary. For example, it may be tempered from the viewpoint of improving the matrix toughness.

EXAMPLE 1

Examples of the steel plate described in (1) of the present invention will be explained next.

Test steel materials of various chemical ingredients were used to produce steel plates of plate thicknesses of 6, 18, 40, and 100 mm under various production conditions. Their properties were evaluated. The evaluated items were, as strength, the yield stress and tensile strength, as toughness, the Charpy impact absorption energy, as the weld heat affected zone toughness in weldability, the Charpy impact absorption energy of the weld joint, and as the machineability, the drilling property. The chemical ingredients, plate thickness, X1, and ferrite fraction measured at various locations of each of the steel plates are shown in Table 1 to Table 6, the production conditions (methods of production 1 to 3) are shown in Table 7 to Table 9, and the results of evaluation of properties are shown in Table 10 to Table 12.

The yield stress and the tensile strength were measured by the metal material tensile test method described in JIS Z 2241. The test piece was a metal material test piece described in JIS Z 2201. No. 5 test pieces taken from steel plates having plate thicknesses of 6 mm and 18 mm, while No. 10 test pieces taken from the t/4 parts of steel plates having plate thicknesses of 40 mm and 100 mm were used. The test pieces were taken so that their longitudinal directions became vertical to the rolling direction. The yield stress was made 0.2% of the yield strength calculated by the lower yield stress or offset method. Two tests were conducted at ordinary temperature and the average values were used.

The matrix toughness was measured by the metal material impact test method described in JIS Z 2242. The test pieces used were the metal material impact test pieces described in JIS Z 2202. For steel plates having thicknesses of 6 mm, subsize test pieces of widths of 5 mm were taken from the plate thickness center parts, for steel plates having thicknesses of 18 mm, test pieces of widths of 10 mm were taken from the plate thickness center parts, while for steel plates having thicknesses of 40 mm and 100 mm, test pieces of widths of 10 mm were taken from the t/4 parts. The shapes were all made V-notch test pieces. The test pieces were taken so that the lines formed by the notch bottoms became parallel to the plate thickness direction and so that the longitudinal directions of the test piece became perpendicular to the rolling direction. The test temperature was made −5° C. The average value of three tests was employed.

For the weld heat affected zone toughness, Charpy test pieces were taken from weld joints prepared by CO₂ gas shield arc welding and submerge arc welding and measured for absorption energy at −5° C. The welding heat input was made 2 to 3 kJ/mm in the case of CO₂ gas shield arc welding and was made 3 kJ/mm for plate thickness 6 mm materials, 5 kJ/mm for plate thickness 18 mm materials, and 7 kJ/mm for plate thickness 40 mm materials and 100 mm materials in the case of submerge arc welding. The test nieces were taken so that the locations 0.5 mm from the weld bond part corresponded to the Charpy test piece notch positions. The average value of three impact absorption energies was used.

The machineability was evaluated by a drilling test using a drilling machine and a high speed drill. The drilling distance was 42 mm in the case of plate thickness 6 mm steel plates piled up in seven layers, 36 mm in the case of plate thickness 18 mm steel plates piled up in two layers, 40 mm in the case of one plate thickness 40 mm steel plate, and 100 mm in the case of one plate thickness 100 mm steel plate for the test. The drill was used to make a through hole using a 6 mmφ diameter high speed drill SKH51. The rotational speed was 1610 rpm, the feed speed was 190 mm/min, and the machining oil was a water soluble machining oil. Under the above conditions, drilling was performed until drilling was no longer possible. The number of holes bored until the limit was reached was measured. TABLE 1 Plate Thickness, Chemical Ingredients, X1, Ferrite Fraction (Method of Production 1) Final plate thick. C Si Mn P S Al N Mo Cr Nb Ti V mm mass % Inv. Ex. 1-1 6 0.13 0.25 0.95 0.0082 0.0043 0.032 0.0035 Comp. Ex. 1-1 6 0.22 0.26 0.55 0.0085 0.0042 0.033 0.0036 Inv. Ex. 1-2 18 0.13 0.55 1.15 0.0038 0.0027 0.036 0.0042 0.008 0.010 Comp. Ex. 1-2 18 0.12 1.20 1.16 0.0048 0.0031 0.032 0.0045 0.008 0.009 Inv. Ex. 1-3 40 0.09 0.75 1.25 0.0054 0.0230 0.033 0.0028 0.15 0.023 Comp. Ex. 1-3 40 0.09 0.74 1.23 0.0051 0.0250 0.033 0.0029 0.15 0.022 Inv. Ex. 1-4 100 0.11 0.44 0.85 0.0055 0.0028 0.025 0.0056 0.23 0.23 Comp. Ex. 1-4 100 0.06 0.42 1.37 0.0068 0.0032 0.033 0.0055 0.19 0.23 Inv. Ex. 1-5 6 0.07 0.46 0.75 0.0083 0.0033 0.023 0.0042 0.25 0.23 0.008 0.018 Comp. Ex. 1-5 6 0.07 0.45 0.78 0.0250 0.0033 0.025 0.0045 0.24 0.22 0.007 0.019 Inv. Ex. 1-6 18 0.06 0.75 0.86 0.0056 0.0043 0.027 0.0033 0.28 0.31 0.015 Comp. Ex. 1-6 18 0.06 0.76 0.45 0.0063 0.0042 0.031 0.0032 1.15 0.32 0.016 Inv. Ex. 1-7 40 0.12 0.15 1.12 0.0058 0.0042 0.035 0.0038 0.35 0.022 Comp. Ex. 1-7 40 0.12 0.16 1.11 0.0062 0.0045 0.041 0.0035 0.34 0.021 Inv. Ex. 1-8 100 0.07 0.65 1.45 0.0035 0.0028 0.026 0.0028 0.05 0.21 Comp. Ex. 1-8 100 0.07 0.66 2.15 0.0040 0.0031 0.032 0.0029 0.01 0.36 Inv. Ex. 1-9 6 0.12 0.32 0.65 0.0085 0.0025 0.018 0.0021 0.15 0.15 0.015 Comp. Ex. 1-9 6 0.12 0.32 0.66 0.0091 0.0031 0.022 0.0120 0.16 0.14 0.016 Inv. Ex. 1-10 18 0.06 0.75 0.95 0.0048 0.0028 0.019 0.0042 0.23 0.12 0.023 Comp. Ex. 1-10 18 0.06 0.76 0.96 0.0058 0.0350 0.022 0.0043 0.23 0.11 0.024 Inv. Ex. 1-11 40 0.08 0.45 1.35 0.0056 0.0025 0.035 0.0029 0.21 Comp. Ex. 1-11 40 0.06 0.44 1.34 0.0061 0.0031 0.120 0.0028 1.15

TABLE 2 (Continuation of Table 1) Cu Ni B REM Ca Zr Mg Ferrite fraction (%) mass % X1 Top 2 mm Rear 2 mm t/4 3t/4 t/2 Inv. Ex. 1-1 0.188 38 36 39 Comp. Ex. 1-1 0.251 34 30 35 Inv. Ex. 1-2 0.201 41 40 42 43 45 Comp. Ex. 1-2 0.222 43 45 48 51 56 Inv. Ex. 1-3 0.186 37 38 40 40 43 Comp. Ex. 1-3 0.187 0 0 0 0 0 Inv. Ex. 1-4 0.5 0.5 0.222 32 31 33 34 36 Comp. Ex. 1-4 0.5 0.6 0.197 0 0 0 0 0 Inv. Ex. 1-5 0.0010 0.160 43 45 52 Comp. Ex. 1-5 0.0011 0.160 42 46 48 Inv. Ex. 1-6 0.0012 0.0013 0.157 48 48 51 52 56 Comp. Ex. 1-6 0.0011 0.0013 0.197 36 34 35 38 39 Inv. Ex. 1-7 0.3 0.4 0.0021 0.215 48 50 52 54 60 Comp. Ex. 1-7 0.3 0.5 0.0022 0.217 25 24 26 28 30 Inv. Ex. 1-8 0.5 0.5 0.0008 0.213 38 37 40 42 45 Comp. Ex. 1-8 0.5 0.5 0.0009 0.253 36 35 37 37 40 Inv. Ex. 1-9 0.176 58 60 72 Comp. Ex. 1-9 0.177 60 63 68 Inv. Ex. 1-10 0.0018 0.151 72 73 75 75 76 Comp. Ex. 1-10 0.0015 0.153 70 70 71 72 72 Inv. Ex. 1-11 0.5 0.5 0.201 71 72 63 64 65 Comp. Ex. 1-11 0.5 0.5 0.228 0 0 12 12 23

TABLE 3 Plate Thickness, Chemical Ingredients, X1, Ferrite Fraction (Method of Production 2) Final plate thick. C Si Mn P S Al N Mo Cr Nb Ti V mm mass % Inv. Ex. 1-12 6 0.09 0.45 0.88 0.0089 0.0032 0.025 0.0038 0.045 Comp. Ex. 1-12 6 0.09 0.46 0.89 0.0091 0.0035 0.032 0.0036 0.112 Inv. Ex. 1-13 18 0.13 0.25 1.25 0.0055 0.0025 0.018 0.0048 0.012 0.010 Comp. Ex. 1-13 18 0.12 0.24 1.16 0.0048 0.0380 0.032 0.0045 0.012 0.011 Inv. Ex. 1-14 40 0.08 0.76 1.15 0.0045 0.0050 0.025 0.0029 0.15 0.25 0.018 Comp. Ex. 1-14 40 0.08 0.75 1.14 0.0050 0.0060 0.033 0.0029 0.15 0.24 0.019 Inv. Ex. 1-15 100 0.11 0.45 0.83 0.0055 0.0025 0.032 0.0045 0.35 0.23 0.008 0.021 Comp. Ex. 1-15 100 0.11 0.43 0.86 0.0056 0.0022 0.023 0.0048 0.34 0.22 0.115 0.022 Inv. Ex. 1-16 6 0.09 0.65 1.56 0.0035 0.0023 0.031 0.0035 Comp. Ex. 1-16 6 0.09 0.66 1.58 0.0041 0.0021 0.032 0.0036 Inv. Ex. 1-17 18 0.12 0.22 0.85 0.0055 0.0042 0.028 0.0021 0.15 0.36 0.011 0.010 0.008 Comp. Ex. 1-17 18 0.12 0.21 0.86 0.0043 0.0042 0.031 0.0022 0.16 0.37 0.012 0.125 0.009

TABLE 4 (Table 3 Continuation) Cu Ni B REM Ca Zr Mg Ferrite fraction (%) mass % X1 Top 2 mm Rear 2 mm t/4 3t/4 t/2 Inv. Ex. 1-12 0.5 0.5 0.186 37 38 40 Comp. Ex. 1-12 0.5 0.5 0.192 63 62 66 Inv. Ex. 1-13 0.202 47 46 50 51 53 Comp. Ex. 1-13 0.190 49 50 55 57 58 Inv. Ex. 1-14 0.0015 0.0018 0.180 72 72 65 60 68 Comp. Ex. 1-14 0.0015 0.0020 0.180 25 26 23 28 32 Inv. Ex. 1-15 0.199 36 37 25 23 28 Comp. Ex. 1-15 0.198 33 30 23 21 25 Inv. Ex. 1-16 0.0018 0.0012 0.185 46 48 51 Comp. Ex. 1-16 0.0018 0.0250 0.187 44 43 50 Inv. Ex. 1-17 0.202 46 43 50 51 55 Comp. Ex. 1-17 0.202 40 38 46 46 49

TABLE 5 Plate Thickness, Chemical Ingredients, X1, Ferrite Fraction (Method of Production 3) Final plate thick. C Si Mn P S Al N Mo Cr Nb Ti V mm mass % Inv. Ex. 1-18 18 0.11 0.25 1.12 0.0038 0.0035 0.032 0.0045 0.15 0.018 Comp. Ex. 1-18 18 0.11 0.26 1.11 0.0041 0.0043 0.032 0.0048 0.16 0.017 Inv. Ex. 1-19 40 0.08 0.55 0.65 0.0035 0.0041 0.032 0.0035 0.55 0.045 Comp. Ex. 1-19 40 0.08 0.56 0.68 0.0041 0.0041 0.032 0.0036 0.56 0.046 Inv. Ex. 1-20 6 0.12 0.76 0.85 0.0061 0.0042 0.031 0.0035 0.012 0.012 Comp. Ex. 1-20 6 0.18 0.76 0.95 0.0061 0.0041 0.032 0.0035 0.012 0.018 Inv. Ex. 1-21 100 0.09 0.25 1.15 0.0023 0.0025 0.035 0.0035 0.23 0.35 Comp. Ex. 1-21 100 0.06 0.26 1.16 0.0023 0.0032 0.033 0.0036 0.23 0.36

TABLE 6 (Table 5 Continuation) Cu Ni B REM Ca Zr Mg Ferrite fraction (%) mass % X1 Top 2 mm Rear 2 mm t/4 3t/4 t/2 Inv. Ex. 1-18 0.2 0.3 0.201 43 46 48 51 55 Comp. Ex. 1-18 1.5 0.3 0.266 41 42 45 48 47 Inv. Ex. 1-19 0.0015 0.0015 0.158 71 72 73 73 74 Comp. Ex. 1-19 0.0230 0.0015 0.161 72 71 68 72 75 Inv. Ex. 1-20 0.190 53 51 54 Comp. Ex. 1-20 0.251 58 58 56 Inv. Ex. 1-21 0.5 0.5 0.221 36 38 39 40 43 Comp. Ex. 1-21 0.5 2.5 0.223 44 45 48 49 52

TABLE 7 Production Conditions (Method of Production 1) Rough rolling Final total Slab plate Heating reduction T1- thickness thickness temperature rate X2 T1 40 T1-80 mm mm ° C. % — ° C. ° C. ° C. Inv. Ex. 1-1 240 6 1150 94 0.053 881 841 801 Comp. Ex. 1-1 240 6 1150 94 0.095 902 862 822 Inv. Ex. 1-2 240 18 1200 81 0.096 858 818 778 Comp. Ex. 1-2 240 18 1200 81 0.207 885 845 805 Inv. Ex. 1-3 240 40 1100 58 0.180 828 788 748 Comp. Ex. 1-3 240 40 1100 25 0.181 828 788 748 Inv. Ex. 1-4 400 100 1100 50 0.509 772 732 692 Comp. Ex. 1-4 400 100 1100 50 0.284 752 712 672 Inv. Ex. 1-5 240 6 1180 92 0.609 967 927 887 Comp. Ex. 1-5 240 6 1180 92 0.564 964 924 884 Inv. Ex. 1-6 240 18 1150 85 0.680 926 886 846 Comp. Ex. 1-6 240 18 1150 85 3.249 981 941 901 Inv. Ex. 1-7 240 40 1250 67 0.183 828 788 748 Comp. Ex. 1-7 240 40 1250 77 0.182 828 788 748 Inv. Ex. 1-8 400 100 1130 50 0.197 739 699 659 Comp. Ex. 1-8 400 100 1130 50 0.150 729 689 649 Inv. Ex. 1-9 240 6 1200 94 0.445 956 916 876 Comp. Ex. 1-9 240 6 1200 94 0.445 956 916 876 Inv. Ex. 1-10 240 18 1100 81 0.463 913 873 833 Comp. Ex. 1-10 240 18 1100 81 0.455 912 872 832 Inv. Ex. 1-11 240 40 1150 58 0.144 820 780 740 Comp. Ex. 1-11 240 40 1150 58 0.495 863 823 783 Finish Water cooling rolling 1st 2nd 1st Total half half pass reduction Flow End cooling cooling Tempering bite rate rate temperature rate rate temperature ° C. % m³/m² · min ° C. ° C./s ° C./s ° C. Inv. Ex. 1-1 862 60 0.7 20 — — 450 Comp. Ex. 1-1 865 60 0.7 20 — — 450 Inv. Ex. 1-2 820 60 1.0 212 — — 550 Comp. Ex. 1-2 850 60 1.0 216 — — 550 Inv. Ex. 1-3 800 60 1.5 156 — — 600 Comp. Ex. 1-3 805 78 1.5 151 — — 600 Inv. Ex. 1-4 735 50 2.0 256 — — 480 Comp. Ex. 1-4 758 50 2.0 262 — — 480 Inv. Ex. 1-5 902 70 0.4 150 5 50 630 Comp. Ex. 1-5 904 70 0.4 152 5 50 630 Inv. Ex. 1-6 853 50 2.0 20 — — 500 Comp. Ex. 1-6 705 50 2.0 20 — — 500 Inv. Ex. 1-7 752 50 1.5 20 — — 480 Comp. Ex. 1-7 755 29 1.5 20 — — 480 Inv. Ex. 1-8 730 50 1.5 375 — — — Comp. Ex. 1-8 722 50 1.5 345 — — — Inv. Ex. 1-9 850 60 0.4 455 — — — Comp. Ex. 1-9 854 60 0.4 625 — — — Inv. Ex. 1-10 800 60 4.0 210 — — 480 Comp. Ex. 1-10 800 60 0.2 218 — — 480 Inv. Ex. 1-11 730 60 1.0 205 — — 630 Comp. Ex. 1-11 755 60 6.0 203 — — 630

TABLE 8 Production Conditions (Method of Production 2) Rough Final rolling Finish rolling Slab plate total 1st pass Total Water cooling thick- thick- Heating reduction bite reduction Start Flow End Tempering ness ness temperature rate temperature rate T2 temperature rate temperature temperature mm mm ° C. % ° C. % ° C. ° C. m³/m² · min ° C. ° C. Inv. Ex. 1-12 240 6 1150 94 820 60 763 755 1.0 200 550 Comp. Ex. 1-12 240 6 1150 94 818 60 762 640 1.0 201 550 Inv. Ex. 1-13 240 18 1200 85 860 50 751 745 1.5 455 — Comp. Ex. 1-13 240 18 1200 85 865 50 760 745 1.5 523 — Inv. Ex. 1-14 240 40 1250 67 912 50 749 720 2.0 20 480 Comp. Ex. 1-14 240 40 1250 67 915 50 750 765 2.0 20 480 Inv. Ex. 1-15 400 100 1250 50 925 50 721 700 1.0 480 — Comp. Ex. 1-15 400 100 1250 67 930 25 720 705 1.0 450 — Inv. Ex. 1-16 240 18 1100 81 905 60 747 735 1.5 200 530 Comp. Ex. 1-16 240 18 1100 81 900 60 745 730 0.2 195 530 Inv. Ex. 1-17 240 40 1200 58 875 60 768 745 1.0 20 550 Comp. Ex. 1-17 240 40 1200 25 878 78 767 740 1.0 20 550

TABLE 9 Production Conditions (Method of Production 3) Rough rolling Finish rolling Slab Final total 1st pass Total Water cooling thick- plate Heating reduction bite reduction Reheating Cooling End Tempering ness thickness temperature rate temperature rate T3 temperature rate temperature temperature mm mm ° C. % ° C. % ° C. ° C. ° C./s ° C. ° C. Inv. Ex. 1-18 240 18 1120 83 905 55 734 755 15 30 450 Comp. Ex. 1-18 240 18 1120 83 915 55 734 870 15 30 450 Inv. Ex. 1-19 240 40 1200 67 860 50 740 760 5 215 450 Comp. Ex. 1-19 240 40 1200 67 870 50 740 710 5 211 450 Inv. Ex. 1-20 100 6 1250 85 915 60 731 760 20 210 500 Comp. Ex. 1-20 100 6 1250 91 918 33 731 755 20 195 500 Inv. Ex. 1-21 400 100 1250 50 885 50 764 780 5 450 — Comp. Ex. 1-21 400 100 1250 25 875 67 764 780 5 520 —

TABLE 10 Results of Evaluation of Properties (Method of Production 1) Weld heat affected zone Weld heat affected zone Tensile Matrix toughness toughness (vE₋₅) toughness (vE₋₅) Number of Yield stress strength (vE₋₅) (CO₂ arc welding) (submerge arc welding) holes MPa MPa J J J No. Inv. Ex. 1-1 504 655 113 94 74 228 Comp. Ex. 1-1 554 698 22 11 5 187 Inv. Ex. 1-2 480 623 175 123 89 278 Comp. Ex. 1-2 514 668 28 23 11 178 Inv. Ex. 1-3 494 625 100 78 65 315 Comp. Ex. 1-3 537 655 82 75 64 15 Inv. Ex. 1-4 478 605 138 88 75 105 Comp. Ex. 1-4 510 615 155 92 69 8 Inv. Ex. 1-5 467 605 101 65 58 423 Comp. Ex. 1-5 487 611 22 16 26 185 Inv. Ex. 1-6 505 623 156 108 100 480 Comp. Ex. 1-6 605 735 25 18 7 26 Inv. Ex. 1-7 515 638 155 123 88 512 Comp. Ex. 1-7 523 655 122 135 75 8 Inv. Ex. 1-8 467 588 215 101 88 111 Comp. Ex. 1-8 550 666 187 5 8 100 Inv. Ex. 1-9 515 630 121 87 61 625 Comp. Ex. 1-9 427 555 17 10 7 380 Inv. Ex. 1-10 489 635 223 208 156 705 Comp. Ex. 1-10 405 552 15 5 8 505 Inv. Ex. 1-11 498 630 123 105 89 380 Comp. Ex. 1-11 586 715 23 32 12 13

TABLE 11 Results of Evaluation of Properties (Method of Production 2) Weld heat affected zone Weld heat affected zone Matrix toughness toughness (vE₋₅) toughness (vE₋₅) No. of Yield stress Tensile strength (vE₋₅) (CO₂ arc welding) (submerge arc welding) holes MPa MPa J J J No. Inv. Ex. 1-12 473 623 81 61 55 203 Comp. Ex. 1-12 423 556 15 13 5 23 Inv. Ex. 1-13 462 608 150 80 72 321 Comp. Ex. 1-13 426 560 21 15 11 418 Inv. Ex. 1-14 476 610 189 89 88 555 Comp. Ex. 1-14 478 625 121 67 78 11 Inv. Ex. 1-15 456 585 156 120 89 191 Comp. Ex. 1-15 535 655 20 21 10 23 Inv. Ex. 1-16 515 625 151 156 123 385 Comp. Ex. 1-16 432 556 25 65 65 156 Inv. Ex. 1-17 608 713 113 78 65 211 Comp. Ex. 1-17 612 725 25 13 12 11

TABLE 12 Results of Evaluation of Properties (Method of Production 3) Weld heat affected zone Weld heat affected zone Matrix toughness toughness (vE₋₅) toughness (vE₋₅) No. of Yield stress Tensile strength (vE₋₅) (CO₂ arc welding) (submerge arc welding) holes MPa MPa J J J No. Inv. Ex. 1-18 512 598 188 155 120 235 Comp. Ex. 1-18 466 581 18 25 15 105 Inv. Ex. 1-19 480 623 135 122 101 565 Comp. Ex. 1-19 475 598 23 82 65 200 Inv. Ex. 1-20 522 628 92 55 51 318 Comp. Ex. 1-20 535 632 21 15 14 158 Inv. Ex. 1-21 465 588 123 89 73 126 Comp. Ex. 1-21 408 545 26 15 18 98

Invention Examples 1-1 to 1-11 are steel plates produced by first method of production (method of production 1), that is, the method of speedily water cooling after rolling (the method of production described in (5) of the present invention). Along with these, Comparative Examples 1-1 to 1-11 are also shown.

Invention Example 1-1 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-1 is similar to Invention Example 1-1 in ingredients and method of production, but the amount of C and X1 are outside the range of the present invention, so the matrix toughness and weld heat affected zone toughness are inferior.

Invention Example 1-2 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 ace steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-2 is similar to Invention Example 1-2 in ingredients and method of production, but the Si is outside the range of the present invention, so the matrix toughness and weld heat affected zone toughness are inferior.

Invention Example 1-3 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 40 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-3 is similar to Invention Example 1-3 in ingredients and method of production, but the total reduction rate of the rough rolling and ferrite fraction are outside the range of the present invention, so the machineability is inferior.

Invention Example 1-4 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 100 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-4 is similar to Invention Example 1-4 in ingredients and method of production, but the finish rolling first pass bite temperature and ferrite fraction are outside the range of the present invention, so the machineability is inferior.

Invention Example 1-5 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-5 is similar to Invention Example 1-5 in ingredients and method of production, but the amount of P is outside the range of the present invention, so the matrix toughness and weld heat affected zone toughness are inferior.

Invention Example 1-6 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-6 is similar to Invention Example 1-6 in ingredients and method of production, but the amount of Mo and the finish rolling first pass bite temperature are outside the range of the present invention, so the matrix toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Example 1-7 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 40 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-7 is similar to Invention Example 1-7 in ingredients and method of production, but the total reduction rate of the finish rolling and ferrite fraction are outside the range of the present invention, so the machineability is inferior.

Invention Example 1-8 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 100 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-8 is similar to Invention Example 1-8 in ingredients and method of production, but the amount of Mn and X1 are outside the range of the present invention, so the weld heat affected zone toughness is inferior.

Invention Example 1-9 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-9 is similar to Invention Example 1-9 in ingredients and method of production, but the amount of N and water cooling end temperature are outside the range of the present invention, so the strength, toughness, and weld heat affected zone toughness are inferior.

Invention Example 1-10 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-10 is similar to Invention Example 1-10 in ingredients and method of production, but the amount of S and flow rate are outside the range of the present invention, so the strength, toughness, and weld heat affected zone toughness are inferior.

Invention Example 1-11 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 40 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-11 is similar to Invention Example 1-11 in ingredients and method of production, but the amount of Al, the amount of Cr, ferrite fraction, and flow rate are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Examples 1-12 to 1-17 are steel plates produced by the second method of production (method of production 2), that is, the method of air cooling until starting to form ferrite after rolling, then water cooling, and the method of production described in (7) of the present invention. Along with these, Comparative Examples 1-12 to 1-17 are also shown.

Invention Example 1-12 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-12 is similar to Invention Example 1-12 in ingredients and method of production, but the amount of V and the water cooling start temperature are outside the range of the present invention, so the strength, toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Example 1-13 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-13 is similar to Invention Example 1-13 in ingredients and method of production, but the water cooling end temperature and the amount of S are outside the range of the present invention, so the strength, toughness, and weld heat affected zone toughness are inferior.

Invention Example 1-14 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 40 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-14 is similar to Invention Example 1-14 in ingredients and method of production, but the water cooling start temperature and ferrite fraction are outside the range of the present invention, so the machineability is inferior.

Invention Example 1-15 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 100 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-15 is similar to Invention Example 1-15 in ingredients and method of production, but the amount of Nb and the total reduction rate of the finish rolling are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Example 1-16 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-16 is similar to Invention Example 1-16 in ingredients and method of production, but the amount of Mg and the flow rate are outside the range of the present invention, so the strength and toughness are inferior.

Invention Example 1-17 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-17 is similar to Invention Example 1-17 in ingredients and method of production, but the amount of Ti and the total reduction rate of the rough rolling are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Examples 1-18 to 1-21 are steel plates produced by the third method of production (method of production 3), that is, the method of heating until the two-phase region again after the temperature the steel plate falls after rolling (the method of production described in (8) of the present invention). Along with these, Comparative Example 1-18 to 1-21 are also shown.

Invention Example 1-18 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 18 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-18 is similar to Invention Example 1-18 in ingredients and method of production, but the amount of Cu, X1, and reheating temperature are outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Example 1-19 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 40 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-19 is similar to Invention Example 1-19 in ingredients and method of production, but the amount of Ca and reheating temperature are outside the range of the present invention, so the toughness is inferior.

Invention Example 1-20 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 6 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-20 is similar to Invention Example 1-20 in ingredients and method of production, but the X1 and the total reduction rate of the finish rolling are outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Example 1-21 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling and water cooling conditions, etc. to produce plate thickness 100 mm steel plate. Ferrite is stably formed in the entire plate thickness range, in particular near the steel plate surface, so this has a tensile strength of 570 MPa or more and is excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 1-21 is similar to Invention Example 1-21 in ingredients and method of production, but the amount of Ni, total reduction rate of the rough rolling, and water cooling end temperature are outside the range of the present invention, so the strength, toughness, and weld heat affected zone toughness are inferior.

From the above examples, it is clear that the steel materials produced by the present invention, that is, the steel plates of Invention Examples 1-1 to 1-21, are steel materials having tensile strengths of 570 to 720 MPa or so, high in matrix toughness, high in weld heat affected zone toughness, and excellent in machineability.

EXAMPLE 2

Examples of the steel plate described in (2) of the present invention will be explained next.

Test steel materials of various chemical ingredients were used to produce steel plates of plate thicknesses of 6, 20, 40, and 100 mm under various production conditions. These were evaluated for, as strength, the yield stress and tensile strength of the matrix, as toughness, the Charpy impact absorption energy of the matrix, as the weld heat affected zone toughness in weldability, the Charpy impact absorption energy of the weld joint, and as the machineability, the drilling property. The chemical ingredients, plate thickness, X1, X2, ferrite fraction, ratio of micro Vickers hardness in a specific range, and Vlckers hardness of each of the steel plates are shown in Table 13 to Table 22, the production conditions (methods of production 4 to 7) are shown in Table 23 to Table 27, and the results of evaluation of properties are shown in Table 28 to Table 32.

The yield stress and the tensile strength were measured by the metal material tensile test method described in JIS Z 2241. The test piece was a metal material test piece described in JIS Z 2201. No. 5 test pieces taken from steel plates having plate thicknesses of 6 mm and 20 mm, while No. 10 test pieces taken from the t/4 parts of steel plates having thicknesses of 40 mm and 100 mm were used. The test pieces were taken so that their longitudinal directions became vertical to the rolling direction. The yield stress was made 0.2% of the yield strength calculated by the lower yield stress or offset method. Two tests were conducted at ordinary temperature and the average values were used.

The matrix toughness was measured by the metal material impact test method described in JIS Z 2242. The test pieces used were the metal material impact test pieces described in JIS Z 2202. For steel plates having thicknesses of 6 mm, subsize test pieces of widths of 5 mm were taken from the plate thickness center parts, for steel plates having thicknesses of 18 mm, test pieces of widths of 10 mm were taken from the plate thickness center parts, while for steel plates having thicknesses of 40 mm and 100 mm, test pieces of widths of 10 mm were taken from the t/4 parts. The shapes were all made V-notch test pieces. The test pieces were taken so that the lines formed by the notch bottoms became parallel to the plate thickness direction and so that the longitudinal directions of the test piece became perpendicular to the rolling direction. The test temperature was made −5° C. The average value of three tests was employed.

For the weld heat affected zone toughness, Charpy test pieces were taken from weld joints prepared by CO₂ gas shield arc welding and submerge arc welding and measured for absorption energy at −5° C. The welding heat input was made 2 to 3 kJ/mm in the case of CO₂ gas shield arc welding and was made 3 kJ/mm for plate thickness 6 mm materials, 5 kJ/mm for plate thickness 20 mm materials, and 7 kJ/mm for plate thickness 40 mm materials and 100 mm materials in the case of submerge arc welding. The test pieces were taken so that the locations 0.5 mm from the weld bond part corresponded to the Charpy test piece notch positions. The average value of three impact absorption energies was used.

The machineability was evaluated by a drilling test using a drilling machine and a high speed drill. The drilling distance was 42 mm in the case of plate thickness 6 mm steel plates piled up in seven layers, 40 mm in the case of plate thickness 20 mm steel plates piled up in two layers, 40 mm in the case of one plate thickness 40 mm steel plate, and 100 mm in the case of one plate thickness 100 mm steel plate for the test. The drill was used to make a through hole using a 6 mmφ diameter high speed drill SKH51. The rotational speed was 1610 rpm, the feed speed was 190 mm/min, and the machining oil was a water soluble machining oil. Under the above conditions, drilling was performed until drilling was no longer possible. The number of holes bored until the limit was reached was measured. TABLE 13 Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 4) Final plate thickness C Si Mn P S Al N Mo Cr Nb Ti V mm mass % Inv. Ex. 2-1 6 0.09 0.85 1.12 0.0081 0.0048 0.031 0.0032 Comp. Ex. 2-1 6 0.09 0.84 1.25 0.0081 0.0050 0.033 0.0033 Inv. Ex. 2-2 6 0.13 0.35 0.85 0.0050 0.0032 0.028 0.0035 0.12 0.18 0.008 Comp. Ex. 2-2 6 0.11 1.20 0.85 0.0051 0.0033 0.029 0.0036 0.11 0.17 0.008 Inv. Ex. 2-3 6 0.10 0.18 0.95 0.0033 0.0029 0.031 0.0028 0.22 0.15 0.020 Comp. Ex. 2-3 6 0.08 0.17 1.55 0.0034 0.0030 0.032 0.0027 0.16 0.15 0.020 Inv. Ex. 2-4 6 0.07 0.31 0.75 0.0066 0.0042 0.015 0.0028 0.30 0.31 Comp. Ex. 2-4 6 0.07 0.32 0.74 0.0065 0.0041 0.015 0.0027 0.31 0.32 Inv. Ex. 2-5 20 0.04 0.43 1.38 0.0039 0.0029 0.035 0.0041 0.05 0.16 0.008 Comp. Ex. 2-5 20 0.04 0.41 1.36 0.0038 0.0028 0.120 0.0042 0.03 0.15 0.007 Inv. Ex. 2-6 20 0.09 0.06 0.85 0.0045 0.0085 0.031 0.0038 0.61 Comp. Ex. 2-6 20 0.09 0.05 0.81 0.0043 0.0110 0.032 0.0039 0.62 Inv. Ex. 2-7 20 0.10 0.35 1.05 0.0065 0.0038 0.011 0.0034 0.25 0.15 0.010 Comp. Ex. 2-7 20 0.10 0.34 1.04 0.0221 0.0036 0.012 0.0035 0.28 0.15 0.010 Inv. Ex. 2-8 20 0.13 0.35 0.71 0.0066 0.0048 0.031 0.0035 0.15 0.20 0.015 Comp. Ex. 2-8 20 0.13 0.36 0.72 0.0067 0.0049 0.032 0.0038 0.16 0.21 0.016 Inv. Ex. 2-9 40 0.16 0.33 1.15 0.0053 0.0029 0.031 0.0029 0.12 0.015 Comp. Ex. 2-9 40 0.16 0.32 1.16 0.0055 0.0030 0.032 0.0030 1.15 0.015 Inv. Ex. 2-10 40 0.08 0.95 1.15 0.0068 0.0045 0.045 0.0045 0.35 Comp. Ex. 2-10 40 0.08 0.96 1.14 0.0067 0.0046 0.046 0.0046 1.13

TABLE 14 (Table 13 Continuation 1) Ratio of micro Vickers Ferrite hardness in specific Vickers Cu Ni B REM Ca Zr Mg fraction range (%) hardness mass % X1 X2 % ≦190HV ≦180HV ≦170HV HV Inv. Ex. 2-1 0.169 0.152 35 195 Comp. Ex. 2-1 0.179 0.134 31 196 Inv. Ex. 2-2 0.202 0.329 50 205 Comp. Ex. 2-2 0.210 0.512 55 203 Inv. Ex. 2-3 0.1 0.2 0.181 0.348 53 38 22 190 Comp. Ex. 2-3 0.2 0.2 0.195 0.174 48 29 18 210 Inv. Ex. 2-4 0.0020 0.148 0.689 70 202 Comp. Ex. 2-4 0.0019 0.152 0.722 0 205 Inv. Ex. 2-5 0.130 0.157 32 15 0 203 Comp. Ex. 2-5 0.129 0.138 30 13 0 200 Inv. Ex. 2-6 0.1 0.1 0.168 0.373 51 192 Comp. Ex. 2-6 0.1 0.1 0.165 0.395 50 191 Inv. Ex. 2-7 0.0015 0.183 0.376 63 196 Comp. Ex. 2-7 0.0018 0.187 0.407 65 198 Inv. Ex. 2-8 0.197 0.451 58 48 41 197 Comp. Ex. 2-8 0.201 0.468 18 0 0 186 Inv. Ex. 2-9 0.233 0.162 46 195 Comp. Ex. 2-9 0.300 1.047 32 254 Inv. Ex. 2-10 0.189 0.317 48 201 Comp. Ex. 2-10 0.227 0.664 46 235

TABLE 15 (Table 13 Continuation 2) Final plate thickness C Si Mn P S Al N Mo Cr Nb Ti V mm mass % Inv. Ex. 2-11 40 0.07 0.38 0.95 0.0055 0.0029 0.032 0.0038 0.32 0.35 0.012 0.008 Comp. Ex. 2-11 40 0.07 0.37 0.94 0.0054 0.0028 0.031 0.0037 0.31 0.36 0.012 0.009 Inv. Ex. 2-12 40 0.13 0.48 0.65 0.0088 0.0035 0.041 0.0045 0.28 0.25 0.010 Comp. Ex. 2-12 40 0.12 0.47 0.66 0.0089 0.0036 0.042 0.0115 0.29 0.26 0.010 Inv. Ex. 2-13 100 0.07 0.45 1.35 0.0055 0.0029 0.015 0.0055 0.51 0.055 0.050 Comp. Ex. 2-13 100 0.07 0.44 1.34 0.0056 0.0028 0.015 0.0055 0.52 0.055 0.051 Inv. Ex. 2-14 100 0.13 0.55 1.15 0.0078 0.0062 0.039 0.0031 0.25 0.15 Comp. Ex. 2-14 100 0.12 0.54 1.14 0.0078 0.0061 0.038 0.0032 0.24 0.16 Inv. Ex. 2-15 100 0.11 0.85 1.05 0.0065 0.0038 0.031 0.0045 0.08 0.25 0.008 0.010 Comp. Ex. 2-15 100 0.11 0.84 1.04 0.0066 0.0039 0.031 0.0046 0.07 0.24 0.012 0.010 Inv. Ex. 2-16 100 0.09 0.37 0.25 0.0055 0.0029 0.025 0.0031 0.45 0.55 0.008 0.015 0.040 Comp. Ex. 2-16 100 0.09 0.37 0.08 0.0054 0.0028 0.026 0.0030 0.55 0.61 0.008 0.014 0.040 Inv. Ex. 2-17 20 0.05 0.44 1.32 0.0038 0.0028 0.035 0.0042 0.07 0.35 0.011 Comp. Ex. 2-17 20 0.04 0.43 1.31 0.0039 0.0027 0.035 0.0041 0.06 0.36 0.012 Inv. Ex. 2-18 20 0.09 0.14 0.85 0.0044 0.0084 0.032 0.0037 0.58 Comp. Ex. 2-18 20 0.08 0.13 0.82 0.0046 0.0085 0.032 0.0038 0.62 Inv. Ex. 2-19 20 0.16 0.35 0.65 0.0065 0.0037 0.035 0.0035 0.18 0.15 0.012 Comp. Ex. 2-19 20 0.21 0.32 0.61 0.0064 0.0036 0.042 0.0036 0.16 0.14 0.010

TABLE 16 (Table 13 Continuation 3) Ratio of micro Vickers Ferrite hardness in specific Cu Ni B REM Ca Zr Mg fraction range (%) Vickers hardness mass % X1 X2 % ≦190HV ≦180HV ≦170HV HV Inv. Ex. 2-11 0.0015 0.164 0.601 55 205 Comp. Ex. 2-11 0.0016 0.164 0.600 58 163 Inv. Ex. 2-12 0.205 0.771 70 198 Comp. Ex. 2-12 0.205 0.779 71 202 Inv. Ex. 2-13 0.5 0.5 0.211 0.256 28 18 5 186 Comp. Ex. 2-13 0.5 0.6 0.214 0.260 0 0 0 188 Inv. Ex. 2-14 0.0010 0.230 0.378 48 192 Comp. Ex. 2-14 0.0075 0.261 0.375 46 195 Inv. Ex. 2-15 0.204 0.357 55 188 Comp. Ex. 2-15 0.203 0.344 13 187 Inv. Ex. 2-16 0.0015 0.0015 0.171 3.196 74 185 Comp. Ex. 2-16 0.0014 0.0013 0.173 11.613 70 184 Inv. Ex. 2-17 0.148 0.252 40 188 Comp. Ex. 2-17 0.146 0.249 42 160 Inv. Ex. 2-18 0.2 0.5 0.180 0.374 53 190 Comp. Ex. 2-18 0.1 0.1 0.167 0.410 0 211 Inv. Ex. 2-19 0.224 0.500 48 34 14 185 Comp. Ex. 2-19 0.269 0.482 47 33 12 210

TABLE 17 Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 5) Final plate thickness C Si Mn P S Al N Mo Cr Nb Ti V mm mass % Inv. Ex. 2-20 6 0.09 0.95 1.20 0.0080 0.0035 0.031 0.0032 0.023 Comp. Ex. 2-20 6 0.08 0.94 1.18 0.0087 0.0038 0.032 0.0033 0.025 Inv. Ex. 2-21 20 0.14 0.65 0.85 0.0075 0.0032 0.033 0.0035 0.11 0.15 Comp. Ex. 2-21 20 0.13 0.66 0.84 0.0073 0.0033 0.035 0.0036 0.11 0.16 Inv. Ex. 2-22 40 0.10 0.35 0.85 0.0053 0.0056 0.034 0.0028 0.25 0.15 0.015 Comp. Ex. 2-22 40 0.10 0.36 0.84 0.0055 0.0055 0.034 0.0027 0.24 0.16 0.110 Inv. Ex. 2-23 100 0.09 0.66 0.95 0.0043 0.0043 0.033 0.0028 0.56 0.018 Comp. Ex. 2-23 100 0.09 0.65 0.94 0.0042 0.0042 0.031 0.0027 0.55 0.112

TABLE 18 (Table 17 Continuation) Ferrite Ratio of micro Vickers Cu Ni B REM Ca Zr Mg fraction hardness in specific range (%) Vickers hardness mass % X1 X2 % ≦190HV ≦180HV ≦170HV HV Inv. Ex. 2-20 0.177 0.158 40 182 Comp. Ex. 2-20 0.174 0.159 20 185 Inv. Ex. 2-21 0.2 0.2 0.227 0.371 35 18 0 173 Comp. Ex. 2-21 0.2 0.3 0.228 0.383 38 25 5 162 Inv. Ex. 2-22 0.180 0.465 53 188 Comp. Ex. 2-22 0.179 0.467 55 189 Inv. Ex. 2-23 0.192 0.728 58 185 Comp. Ex. 2-23 0.190 0.723 66 162

TABLE 19 Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 6) Final plate thickness C Si Mn P S Al N Mo Cr Nb Ti V mm mass % Inv. Ex. 2-24 6 0.14 0.85 1.05 0.0085 0.0035 0.031 0.0030 Comp. Ex. 2-24 6 0.13 0.84 1.06 0.0086 0.0036 0.033 0.0031 Inv. Ex. 2-25 20 0.11 0.35 1.05 0.0074 0.0034 0.013 0.0035 0.25 0.023 Comp. Ex. 2-25 20 0.11 0.36 1.04 0.0075 0.0033 0.013 0.0034 0.24 0.024 Inv. Ex. 2-26 40 0.07 0.38 0.85 0.0065 0.0028 0.035 0.0032 0.75 0.020 Comp. Ex. 2-26 40 0.06 0.39 0.85 0.0064 0.0025 0.034 0.0033 0.74 0.120 Inv. Ex. 2-27 100 0.12 0.68 0.88 0.0053 0.0055 0.041 0.0045 0.21 0.25 Comp. Ex. 2-27 100 0.12 0.67 0.87 0.0054 0.0054 0.042 0.0043 0.22 0.24 Inv. Ex. 2-28 20 0.09 0.15 0.65 0.0032 0.0035 0.045 0.0023 0.53 Comp. Ex. 2-28 20 0.09 0.14 0.66 0.0035 0.0032 0.052 0.0025 0.54 Inv. Ex. 2-29 100 0.07 0.25 0.65 0.0065 0.0033 0.035 0.0033 0.35 0.75 Comp. Ex. 2-29 100 0.06 0.24 0.64 0.0066 0.0034 0.034 0.0035 0.36 0.74

TABLE 20 (Table 19 Continuation) Ferrite Ratio of micro Vickers Cu Ni B REM Ca Zr Mg fraction hardness in specific range (%) Vickers hardness mass % X1 X2 % ≦190HV ≦180HV ≦170HV HV Inv. Ex. 2-24 0.216 0.162 25 10 0 180 Comp. Ex. 2-24 0.215 0.158 0 0 0 182 Inv. Ex. 2-25 0.0015 0.191 0.305 55 185 Comp. Ex. 2-25 0.0205 0.191 0.300 60 162 Inv. Ex. 2-26 0.160 0.531 62 199 Comp. Ex. 2-26 0.169 0.527 61 164 Inv. Ex. 2-27 0.2 0.3 0.229 0.535 66 201 Comp. Ex. 2-27 0.2 2.1 0.260 0.545 48 227 Inv. Ex. 2-28 0.161 0.862 55 185 Comp. Ex. 2-28 0.160 0.861 67 160 Inv. Ex. 2-29 0.3 0.182 1.192 72 182 Comp. Ex. 2-29 1.1 0.220 1.216 73 191

TABLE 21 Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 7) Final plate thickness C Si Mn P S Al N Mo Cr Nb Ti V mm mass % Inv. Ex. 2-30 6 0.13 0.25 0.95 0.0067 0.0055 0.031 0.0035 0.25 Comp. Ex. 2-30 6 0.13 0.24 0.94 0.0066 0.0054 0.032 0.0038 0.24 Inv. Ex. 2-31 20 0.11 0.33 0.85 0.0055 0.0033 0.033 0.0045 0.31 Comp. Ex. 2-31 20 0.10 0.32 0.84 0.0054 0.0032 0.033 0.0042 0.32 Inv. Ex. 2-32 40 0.08 0.35 0.55 0.0087 0.0044 0.034 0.0043 0.42 0.43 0.008 0.010 Comp. Ex. 2-32 40 0.08 0.34 0.54 0.0086 0.0045 0.032 0.0041 0.41 0.42 0.008 0.011 Inv. Ex. 2-33 100 0.12 0.33 0.98 0.0077 0.0043 0.033 0.0042 0.35 0.15 0.040 Comp. Ex. 2-33 100 0.11 0.32 0.99 0.0075 0.0042 0.031 0.0040 0.34 0.14 0.112 Inv. Ex. 2-34 100 0.08 0.25 0.75 0.0077 0.0031 0.045 0.0035 0.85 0.035 Comp. Ex. 2-34 100 0.08 0.24 0.74 0.0074 0.0030 0.052 0.0030 0.84 0.110

TABLE 22 (Table 21 Continuation) Ferrite Ratio of micro Vickers Vickers Cu Ni B REM Ca Zr Mg fraction hardness in specific range (%) hardness mass % X1 X2 % ≦190HV ≦180HV ≦170HV HV Inv. Ex. 2-30 0.0022 0.200 0.184 25 0 0 185 Comp. Ex. 2-30 0.1210 0.198 0.179 25 0 0 183 Inv. Ex. 2-31 0.0023 0.179 0.442 55 182 Comp. Ex. 2-31 0.0250 0.178 0.457 54 164 Inv. Ex. 2-32 0.0020 0.171 1.282 61 185 Comp. Ex. 2-32 0.0215 0.168 1.274 68 163 Inv. Ex. 2-33 0.210 0.501 58 190 Comp. Ex. 2-33 0.215 0.479 56 191 Inv. Ex. 2-34 0.180 1.200 61 186 Comp. Ex. 2-34 0.177 1.200 63 187

TABLE 23 Production Conditions (Method of Production 4) Rough rolling Final total Slab plate Heating reduction thickness thickness temperature rate X2 T4 T4 - 40 T4 - 80 mm mm ° C. % — ° C. ° C. ° C. Inv. Ex. 2-1 240 6 1100 88 0.152 949 909 869 Comp. Ex. 2-1 240 6 1100 88 0.134 944 904 864 Inv. Ex. 2-2 240 6 1150 88 0.329 976 936 896 Comp. Ex. 2-2 240 6 1150 88 0.512 991 951 911 Inv. Ex. 2-3 240 6 1150 88 0.348 978 938 898 Comp. Ex. 2-3 240 6 1150 88 0.174 953 913 873 Inv. Ex. 2-4 240 6 1200 88 0.689 1001 961 921 Comp. Ex. 2-4 240 6 1200 88 0.722 1003 963 923 Inv. Ex. 2-5 240 20 1180 83 0.157 899 859 819 Comp. Ex. 2-5 240 20 1180 83 0.138 894 854 814 Inv. Ex. 2-6 240 20 1150 81 0.373 929 889 849 Comp. Ex. 2-6 240 20 1150 81 0.395 931 891 851 Inv. Ex. 2-7 240 20 1100 79 0.376 930 890 850 Comp. Ex. 2-7 240 20 1100 79 0.407 932 892 852 Inv. Ex. 2-8 240 20 1100 79 0.451 936 896 856 Comp. Ex. 2-8 240 20 1100 79 0.468 937 897 857 Inv. Ex. 2-9 240 40 1200 58 0.162 854 814 774 Comp. Ex. 2-9 240 40 1200 58 1.047 919 879 839 Inv. Ex. 2-10 240 40 1100 67 0.317 877 837 797 Comp. Ex. 2- 240 40 1100 67 0.664 903 863 823 10 Finish water cooling rolling 1st 2nd Total half half 1st pass reduction Flow End cooling cooling Tempering bite rate rate temperature rate rate temperature ° C. % m³/m² · min ° C. ° C./s ° C./s ° C. Inv. Ex. 2-1 924 80 1 215 — — 500 Comp. Ex. 2-1 926 80 1 210 — — 500 Inv. Ex. 2-2 905 80 1 188 — — 550 Comp. Ex. 2-2 915 80 1 195 — — 550 Inv. Ex. 2-3 876 80 0.3 188 — — 450 Comp. Ex. 2-3 870 80 0.3 201 — — 450 Inv. Ex. 2-4 905 80 2 156 — — 600 Comp. Ex. 2-4 1015 80 2 162 — — 600 Inv. Ex. 2-5 890 50 2 211 — — 550 Comp. Ex. 2-5 891 50 2 215 — — 550 Inv. Ex. 2-6 865 56 1 455 — — — Comp. Ex. 2-6 860 56 1 460 — — — Inv. Ex. 2-7 838 60 3 103 — — 600 Comp. Ex. 2-7 852 60 3 100 — — 600 Inv. Ex. 2-8 850 60 1 212 — — 550 Comp. Ex. 2-8 720 60 1 195 — — 550 Inv. Ex. 2-9 822 60 1 155 — — 550 Comp. Ex. 2-9 885 60 1 150 — — 550 Inv. Ex. 2-10 791 50 2 485 — — — Comp. Ex. 2- 802 50 2 475 — — — 10

TABLE 24 (Table 23 Continuation) Rough rolling Final total Slab plate Heating reduction thickness thickness temperature rate X2 T4 T4 - 40 T4 - 80 mm mm ° C. % — ° C. ° C. ° C. Inv. Ex. 2-11 240 40 1150 71 0.601 900 860 820 Comp. Ex. 2-11 240 40 1150 71 0.600 900 860 820 Inv. Ex. 2-12 240 40 1000 45 0.771 909 869 829 Comp. Ex. 2-12 240 40 1000 45 0.779 909 869 829 Inv. Ex. 2-13 400 100 1100 50 0.256 778 738 698 Comp. Ex. 2-13 400 100 1100 66 0.260 779 739 699 Inv. Ex. 2-14 400 100 1200 50 0.378 792 752 712 Comp. Ex. 2-14 400 100 1200 50 0.375 791 751 711 Inv. Ex. 2-15 400 100 1200 50 0.357 790 750 710 Comp. Ex. 2-15 400 100 1200 25 0.344 788 748 708 Inv. Ex. 2-16 400 100 1220 50 3.196 866 826 786 Comp. Ex. 2-16 400 100 1220 50 11.613 912 872 832 Inv. Ex. 2-17 240 20 1150 83 0.252 916 876 836 Comp. Ex. 2-17 240 20 1150 83 0.249 915 875 835 Inv. Ex. 2-18 240 20 1150 81 0.374 930 890 850 Comp. Ex. 2-18 240 20 1150 81 0.410 933 893 853 Inv. Ex. 2-19 240 20 1180 79 0.500 940 900 860 Comp. Ex. 2-19 240 20 1180 79 0.482 938 898 858 Finish Water cooling rolling 1st 2nd Total half half 1st pass reduction Flow End cooling cooling Tempering bite rate rate temperature rate rate temperature ° C. % m³/m² · min ° C. ° C./s ° C./s ° C. Inv. Ex. 2-11 825 43 3 210 — — 580 Comp. Ex. 2-11 822 43 0.1 200 — — 580 Inv. Ex. 2-12 818 70 0.5/2 155 5 25 500 Comp. Ex. 2-12 820 70 0.5/2 156 5 25 500 Inv. Ex. 2-13 768 50 1 218 — — 450 Comp. Ex. 2-13 760 26 1 225 — — 450 Inv. Ex. 2-14 780 50 1 105 — — 450 Comp. Ex. 2-14 785 50 1 108 — — 450 Inv. Ex. 2-15 740 50 2 201 — — 450 Comp. Ex. 2-15 743 67 2 202 — — 450 Inv. Ex. 2-16 775 50 1 156 — — 500 Comp. Ex. 2-16 825 50 1 134 — — 500 Inv. Ex. 2-17 910 50 1 585 — — — Comp. Ex. 2-17 911 50 1 612 — — — Inv. Ex. 2-18 862 56 1 20 — — 450 Comp. Ex. 2-18 860 56 6 20 — — 450 Inv. Ex. 2-19 830 60 1 103 — — 500 Comp. Ex. 2-19 840 60 1 100 — — 500

TABLE 25 Production Conditions (Method of Production 5) Final Reheating Cooling Tempering Slab thickness plate thickness temperature Cooling rate end temperature temperature mm mm ° C. ° C./s ° C. ° C. Inv. Ex. 2-20 240 6 920 30 150 480 Comp. Ex. 2-20 240 6 1080 30 155 480 Inv. Ex. 2-21 240 20 950 20 475 450 Comp. Ex. 2-21 240 20 950 20 523 450 Inv. Ex. 2-22 240 40 1000 15 188 550 Comp. Ex. 2-22 240 40 880 15 158 550 Inv. Ex. 2-23 240 100 930 3 455 — Comp. Ex. 2-23 240 100 930 0.5 458 —

TABLE 26 Production Conditions (Method of Production 6) Rough Finish rolling Water cooling rolling 1st pass Total Flow Slab Final plate Heating total bite reduction Start rate End Tempering thickness thickness temperature reduction temperature rate temperature m³/ temperature temperature mm mm ° C. rate % ° C. % ° C. m² · min ° C. ° C. Inv. Ex. 2-24 240 6 1050 88 865 80 715 1 201 550 Comp. Ex. 2-24 240 6 1050 88 860 80 780 1 222 550 Inv. Ex. 2-25 240 20 1150 83 912 50 725 1.5 480 — Comp. Ex. 2-25 240 20 1150 83 915 50 728 1.5 550 — Inv. Ex. 2-26 240 40 1250 67 880 50 721 2 55 550 Comp. Ex. 2-26 240 40 1250 67 870 50 650 2 78 550 Inv. Ex. 2-27 400 100 1280 50 878 50 743 1 20 500 Comp. Ex. 2-27 400 100 1280 65 895 29 742 1 20 500 Inv. Ex. 2-28 240 20 1150 75 911 67 733 3 20 480 Comp. Ex. 2-28 240 20 1150 75 910 67 732 0.1 20 480 Inv. Ex. 2-29 400 100 1180 50 850 50 745 1.5 20 450 Comp. Ex. 2-29 400 100 1180 25 845 67 742 1.5 20 450

TABLE 27 Production Conditions (Method of Production 7) Rough Finish rolling Final rolling 1st pass Total Water cooling Slab plate Heating total bite reduction Reheating End Tempering thickness thickness temperature reduction temperature rate temperature Cooling temperature temperature mm mm ° C. rate % ° C. % ° C. rate ° C./s ° C. ° C. Inv. Ex. 2-30 240 6 1120 92 925 70 770 40 125 620 Comp. Ex. 2-30 240 6 1120 92 920 70 680 40 105 620 Inv. Ex. 2-31 240 20 1150 79 885 60 780 25 455 — Comp. Ex. 2-31 240 20 1150 79 886 60 785 25 535 — Inv. Ex. 2-32 240 40 1200 50 912 67 820 5 20 550 Comp. Ex. 2-32 240 40 1200 50 911 67 915 0.5 20 550 Inv. Ex. 2-33 400 100 1150 50 868 50 750 5 395 — Comp. Ex. 2-33 400 100 1150 65 870 29 750 5 425 — Inv. Ex. 2-34 400 100 1200 50 858 50 800 3 20 530 Comp. Ex. 2-34 400 100 1200 25 860 67 800 3 20 530

TABLE 28 Results of Evaluation of Properties (Method of Production 4) Weld heat affected zone Weld heat affected zone Matrix toughness toughness (vE₋₅) toughness (vE₋₅) Yield stress Tensile strength (vE₋₅) (CO₂ arc welding) (submerge arc welding) No. of holes MPa MPa J J J No. Inv. Ex. 2-1 491 638 107 73 63 235 Comp. Ex. 2-1 522 645 90 61 53 32 Inv. Ex. 2-2 504 655 128 61 55 413 Comp. Ex. 2-2 525 648 11 9 4 315 Inv. Ex. 2-3 475 589 96 73 63 678 Comp. Ex. 2-3 567 689 86 13 11 205 Inv. Ex. 2-4 501 638 97 76 56 845 Comp. Ex. 2-4 529 645 92 68 50 18 Inv. Ex. 2-5 495 643 186 125 103 255 Comp. Ex. 2-5 497 645 20 68 55 10 Inv. Ex. 2-6 472 613 123 85 78 413 Comp. Ex. 2-6 508 620 23 18 15 455 Inv. Ex. 2-7 502 635 256 178 138 635 Comp. Ex. 2-7 518 632 41 18 9 545 Inv. Ex. 2-8 505 623 245 168 105 898 Comp. Ex. 2-8 468 583 205 138 76 23 Inv. Ex. 2-9 488 618 178 89 88 225 Comp. Ex. 2-9 689 811 135 22 23 232 Inv. Ex. 2-10 480 608 189 123 115 413 Comp. Ex. 2-10 643 756 168 27 33 400

TABLE 29 (Table 28 Continuation) Weld heat affected zone Weld heat affected zone Matrix toughness toughness (vE₋₅) toughness (vE₋₅) Yield stress Tensile strength (vE₋₅) (CO₂ arc welding) (submerge arc welding) No. of holes MPa MPa J J J No. Inv. Ex. 2-11 478 605 168 108 88 623 Comp. Ex. 2-11 425 535 155 98 75 555 Inv. Ex. 2-12 484 613 289 105 88 815 Comp. Ex. 2-12 505 623 20 23 28 612 Inv. Ex. 2-13 472 598 125 103 88 111 Comp. Ex. 2-13 488 588 88 78 77 8 Inv. Ex. 2-14 480 608 178 79 65 213 Comp. Ex. 2-14 505 623 13 13 8 159 Inv. Ex. 2-15 493 632 168 133 78 312 Comp. Ex. 2-15 485 631 135 120 66 13 Inv. Ex. 2-16 463 593 232 205 166 423 Comp. Ex. 2-16 459 589 198 25 13 310 Inv. Ex. 2-17 485 572 212 138 113 124 Comp. Ex. 2-17 420 535 178 121 98 101 Inv. Ex. 2-18 485 620 138 102 79 213 Comp. Ex. 2-18 525 670 130 100 68 10 Inv. Ex. 2-19 501 630 120 85 88 355 Comp. Ex. 2-19 525 660 18 27 25 273

TABLE 30 Results of Evaluation of Properties (Method of Production 5) Weld heat affected Weld heat affected zone zone Matrix toughness toughness (vE₋₅) toughness (vE₋₅) Yield stress Tensile strength (vE₋₅) (CO₂ arc welding) (submerge arc welding) No. of holes MPa MPa J J J No. Inv. Ex. 2-20 481 625 80 50 45 205 Comp. Ex. 2-20 511 628 83 46 43 13 Inv. Ex. 2-21 454 589 235 89 70 413 Comp. Ex. 2-21 420 555 201 80 68 315 Inv. Ex. 2-22 450 585 188 218 155 432 Comp. Ex. 2-22 479 622 18 15 14 18 Inv. Ex. 2-23 476 588 180 105 88 257 Comp. Ex. 2-23 460 558 15 18 13 14

TABLE 31 Results of Evaluation of Properties (Method of Production 6) Weld heat affected zone Weld heat affected zone Matrix toughness toughness (vE₋₅) toughness (vE₋₅) Yield stress Tensile strength (vE₋₅) (CO₂ arc welding) (submerge arc welding) No. of holes MPa MPa J J J No. Inv. Ex. 2-24 466 613 76 50 36 203 Comp. Ex. 2-24 483 620 93 45 32 29 Inv. Ex. 2-25 464 611 212 156 130 411 Comp. Ex. 2-25 412 538 23 88 68 356 Inv. Ex. 2-26 493 632 193 120 88 611 Comp. Ex. 2-26 468 543 23 18 15 8 Inv. Ex. 2-27 493 632 188 156 110 407 Comp. Ex. 2-27 613 718 26 25 20 306 Inv. Ex. 2-28 515 645 211 110 88 615 Comp. Ex. 2-28 414 520 156 105 66 380 Inv. Ex. 2-29 467 586 158 123 77 406 Comp. Ex. 2-29 512 638 32 22 15 358

TABLE 32 Results of Evaluation of Properties (Method of Production 7) Weld heat affected zone Weld heat affected zone Matrix toughness toughness (vE₋₅) toughness (vE₋₅) Yield stress Tensile strength (VE₋₅) (CO₂ arc welding) (submerge arc welding) No. of holes MPa MPa J J J No. Inv. Ex. 2-30 497 645 88 46 41 223 Comp. Ex. 2-30 488 630 17 46 41 21 Inv. Ex. 2-31 481 625 125 88 69 411 Comp. Ex. 2-31 423 550 18 67 75 310 Inv. Ex. 2-32 460 598 185 158 128 612 Comp. Ex. 2-32 420 555 23 101 95 312 Inv. Ex. 2-33 476 618 168 98 69 209 Comp. Ex. 2-33 532 678 23 23 30 5 Inv. Ex. 2-34 478 608 210 153 110 309 Comp. Ex. 2-34 540 656 29 18 15 8

Invention Examples 2-1 to 2-19 are steel plates produced by first method of production of steel plates described in (2) of the present invention (method of production 4), that is, the method of speedily water cooling after rolling, and the method of production described in (9) of the present invention. Along with these, Comparative Examples 2-1 to 2-19 are also shown.

Invention Example 2-1 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-1 is similar to Invention Example 2-1 in ingredients and method of production, but X2 is outside the range of the present invention, so the machineability is inferior.

Invention Example 2-2 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-2 is similar to Invention Example 2-2 in ingredients and method of production, but the amount of Si is outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Example 2-3 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-3 is similar to Invention Example 2-3 in ingredients and method of production, the amount of Mn is outside the range of the present invention, so the weld heat affected zone toughness is inferior.

Invention Example 2-4 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-4 is similar to Invention Example 2-4 in ingredients and method of production, but the finish rolling first pass bite temperature and ferrite fraction are outside the range of the present invention, so the machineability is inferior.

Invention Example 2-5 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-5 is similar to Invention Example 2-5 in ingredients and method of production, but the amount of A1 and X2 are outside the range of the present invention, so the toughness is inferior.

Invention Example 2-6 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-6 is similar to Invention Example 2-6 in ingredients and method of production, but the amount of S is outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Example 2-7 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-7 is similar to Invention Example 2-7 in ingredients and method of production, but the amount of P is outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Example 2-8 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-8 is similar to Invention Example 2-8 in ingredients and method of production, but the finish rolling first pass bite temperature and the micro Vickers hardness are outside the range of the present invention, so the machineability is inferior.

Invention Example 2-9 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-9 is similar to Invention Example 2-9 in ingredients and method of production, but the amount of Mo and the X1 are outside the range of the present invention, so the weld heat affected zone toughness is inferior.

Invention Example 2-10 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-10 is similar to Invention Example 2-10 in ingredients and method of production, but the amount of Cr is outside the range of the present invention, so the weld heat affected zone toughness is inferior.

Invention Example 2-11 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-11 is similar to Invention Example 2-11 in ingredients and method of production, but the Vickers hardness and flow rate are outside the range of the present invention, so the strength is inferior.

Invention Example 2-12 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-12 is similar to Invention Example 2-12 in ingredients and method of production, but the amount of N is outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Example 2-13 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-13 is similar to Invention Example 2-13 in ingredients and method of production, but the total reduction rate of the finish rolling and the micro Vickers hardness are outside the range of the present invention, so the machineability is inferior.

Invention Example 2-14 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-14 is similar to Invention Example 2-14 in ingredients and method of production, but the amount of B and the X1 are outside the range of the present invention, so the toughness is inferior.

Invention Example 2-15 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-15 is similar to Invention Example 2-15 in ingredients and method of production, but the ferrite fraction and the total reduction rate of the rough rolling are outside the range of the present invention, so the machineability is inferior.

Invention Example 2-16 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-16 is similar to Invention Example 2-16 in ingredients and method of production, but the X2 is outside the range of the present invention, so the weld heat affected zone toughness is inferior.

Invention Example 2-17 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-17 is similar to Invention Example 2-17 in ingredients and method of production, but the water cooling end temperature and the Vickers hardness are outside the range of the present invention, so the strength is inferior.

Invention Example 2-18 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-18 is similar to Invention Example 2-18 in ingredients and method of production, but the flow rate and ferrite fraction are outside the range of the present invention, so the machineability is inferior.

Invention Example 2-19 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-19 is similar to Invention Example 2-19 in ingredients and method of production, but the amount of C and the X1 are outside the range of the present invention, so the toughness and the weld heat affected zone toughness are inferior.

Invention Examples 2-20 to 2-23 are steel plates produced by the second method of production of steel plates described in (2) of the present invention (method of production 5), that is, the method of reheating after the temperature of the steel plate falls after rolling, then water cooling, and the method of production described in (11) of the present invention. Along with these, Comparative Examples 2-20 to 2-23 are also shown.

Invention Example 2-20 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-20 is similar to Invention Example 2-20 in ingredients and method of production, but the reheating temperature and ferrite fraction are outside the range of the present invention, so the machineability is inferior.

Invention Example 2-21 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-21 is similar to Invention Example 2-21 in ingredients and method of production, but the water cooling end temperature after reheating and the Vickers hardness are outside the range of the present invention, so the strength is inferior.

Invention Example 2-22 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-22 is similar to Invention Example 2-22 in ingredients and method of production, but the amount of Nb and the reheating temperature are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Example 2-23 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-23 is similar to Invention Example 2-23 in ingredients and method of production, but the amount of Ti, cooling rate after reheating, and Vickers hardness are outside the range of the present invention, so the strength, toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Examples 2-24 to 2-29 are steel plates produced by the third method of production of steel plates described in (2) of the present invention (method of production 6), that is, the method of air cooling until starting to form ferrite after rolling, then water cooling, and the method of production described in (12) of the present invention. Along with these, Comparative Examples 2-24 to 2-29 are also shown.

Invention Example 2-24 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-24 is similar to Invention Example 2-24 in ingredients and method of production, but the water cooling start temperature and the micro Vickers hardness are outside the range of the present invention, so the machineability is inferior.

Invention Example 2-25 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-25 is similar to Invention Example 2-25 in ingredients and method of production, but the amount of Zr, water cooling end temperature, and Vickers hardness are outside the range of the present invention, so the strength and toughness are inferior.

Invention Example 2-26 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability arc achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-26 is similar to Invention Example 2-26 in ingredients and method of production, but the amount of V, water cooling start temperature, and Vickers hardness are outside the range of the present invention, so the strength, toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Example 2-27 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-27 is similar to Invention Example 2-27 in ingredients and method of production, but the amount of Ni, X1, and the total reduction rate of the finish rolling are outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Example 2-28 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-28 is similar to Invention Example 2-28 in ingredients and method of production, but the flow rate and Vickers hardness are outside the range of the present invention, so the strength is inferior.

Invention Example 2-29 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-29 is similar to Invention Example 2-29 in ingredients and method of production, but the amount of Cu and the total reduction rate of the rough rolling are outside the range of the present invention, so the toughness and weld heat affected zone toughness are inferior.

Invention Examples 2-30 to 2-34 are steel plates produced by the fourth method of production of steel plates described in (2) of the present invention (method of production 7), that is, the method of heating until the two-phase region again after the temperature the steel plate falls after rolling and the method of production described in (13) of the present invention. Along with these, Comparative Examples 2-30 to 2-34 are also shown.

Invention Example 2-30 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 6 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-30 is similar to Invention Example 2-30 in ingredients and method of production, but the amount of RPM and reheating temperature are outside the range of the present invention, so the toughness and the machineability are inferior.

Invention Example 2-31 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 20 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-31 is similar to Invention Example 2-31 in ingredients and method of production, but the amount of Ca, the water cooling end temperature after reheating, and the Vickers hardness are outside the range of the present invention, so the strength and toughness are inferior.

Invention Example 2-32 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 40 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-32 is similar to Invention Example 2-32 in ingredients and method of production, but the amount of Mg, reheating temperature, and Vickers hardness are outside the range of the present invention, so the strength and toughness are inferior.

Invention Example 2-33 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-33 is similar to Invention Example 2-33 in ingredients and method of production, but the amount of V and the total reduction rate of the finish rolling are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.

Invention Example 2-34 is tensile strength 570 MPa or so steel plate where a high toughness, weldability, and machineability are achieved by a method of production controlling the balance of alloying ingredients, rolling conditions, water cooling conditions, etc. to produce plate thickness 100 mm steel plate having a tensile strength of 570 MPa or more and excellent in matrix toughness, weld heat affected zone toughness, and machineability. On the other hand, Comparative Example 2-34 is similar to Invention Example 2-34 in ingredients and method of production, but the amount of Nb and the total reduction rate of the rough rolling are outside the range of the present invention, so the toughness, weld heat affected zone toughness, and machineability are inferior.

From the above examples, it is clear that the steel materials produced by the present invention, that is, the steel plates of Invention Examples 2-1 to 2-34, are steel materials having tensile strengths of 570 to 720 MPa or so and excellent in all of toughness, weldability, and machineability.

EXAMPLE 3

Examples of the steel plate described in (3) of the present invention will be explained next.

Test steel materials of various chemical ingredients were used to produce steel plates of plate thicknesses of 6, 18, 40, and 100 mm under various production conditions. These were evaluated for, as strength, the yield stress and tensile strength of the matrix, as toughness, the Charpy impact absorption energy of the matrix, and as the machineability, the drilling property. The chemical ingredients, plate thickness, X1, X2, ferrite fraction, Vickers hardness, and ratio of micro Vickers hardness in a specific range of each of the steel plates are shown in Table 33 to Table 40, the production conditions (methods of production 4′ to 7′) are shown in Table 41 to Table 44, and the results of evaluation of properties are shown in Table 45 to Table 48.

The yield stress and the tensile strength were measured by the metal material tensile test method described in JIS Z 2241. The test piece was a metal material test piece described in JIS Z 2201. No. 5 test pieces taken from steel plates having thicknesses of 6 mm and 18 mm, while No. 10 test pieces taken from the t/4 parts of steel plates having thicknesses of 40 mm and 100 mm were used. The test pieces were taken so that their longitudinal directions became vertical to the rolling direction. The yield stress was made 0.2% of the yield strength calculated by the lower yield stress or offset method. Two tests were conducted at ordinary temperature and the average values were used.

The matrix toughness was measured by the metal material impact test method described in JIS Z 2242. The test pieces used were the metal material impact test pieces described in JIS Z 2202. For steel plates having thicknesses of 6 mm, subsize test pieces of widths of 5 mm were taken from the plate thickness center parts, for steel plates having thicknesses of 18 mm, test pieces of widths of 10 mm were taken from the plate thickness center parts, while for steel plates having thicknesses of 40 mm and 100 mm, test pieces of widths of 10 mm were taken from the t/4 parts. The shapes were all made V-notch test pieces. The test pieces were taken so that the lines formed by the notch bottoms became parallel to the plate thickness direction and so that the longitudinal directions of the test piece became perpendicular to the rolling direction. The test temperature was made −5° C. The average value of three tests was employed.

The machineability was evaluated by a drilling test using a drilling machine and a high speed drill. The drilling distance was 42 mm in the case of plate thickness 6 mm steel plates piled up in seven layers, 36 mm in the case of plate thickness 18 mm steel plates piled up in two layers, 40 mm in the case of one plate thickness 40 mm steel plate, and 100 mm in the case of one plate thickness 100 mm steel plate for the test. The drill was used to make a through hole using a 6 mmφ diameter high speed drill SKH51. The rotational speed was 1610 rpm, the feed speed was 190 mm/min, and the machining oil was a water soluble machining oil. Under the above conditions, drilling was performed until drilling was no longer possible. The number of holes bored until the limit was reached was measured. TABLE 33 Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 4′) Final plate thickness C Si Mn P S Al N Mo Cr Nb Ti V mm mass % Inv. Ex. 3-1 18 0.18 0.58 0.72 0.0068 0.018 0.031 0.0035 Comp. Ex. 3-1 18 0.22 0.56 0.71 0.0069 0.017 0.032 0.0036 Inv. Ex. 3-2 18 0.12 0.15 0.85 0.0075 0.031 0.033 0.0045 0.23 Comp. Ex. 3-2 18 0.12 0.14 0.84 0.0076 0.032 0.033 0.0043 0.22 Inv. Ex. 3-3 18 0.13 0.35 0.71 0.0080 0.025 0.031 0.0029 0.16 0.22 0.016 Comp. Ex. 3-3 18 0.13 0.36 1.35 0.0081 0.026 0.032 0.0028 0.015 Inv. Ex. 3-4 6 0.06 0.75 1.12 0.0045 0.031 0.018 0.0051 0.25 Comp. Ex. 3-4 6 0.06 1.15 1.15 0.0043 0.033 0.021 0.0050 0.21 Inv. Ex. 3-5 6 0.14 0.35 0.88 0.0038 0.019 0.028 0.0030 0.07 0.30 Comp. Ex. 3-5 6 0.14 0.34 0.89 0.0036 0.019 0.032 0.0031 0.07 0.31 Inv. Ex. 3-6 6 0.12 0.40 0.65 0.0028 0.012 0.025 0.0025 0.12 0.22 0.015 Comp. Ex. 3-6 6 0.11 0.41 0.68 0.0056 0.013 0.033 0.0026 0.12 0.21 0.014 Inv. Ex. 3-7 40 0.15 0.88 1.12 0.0045 0.033 0.018 0.0021 0.018 Comp. Ex. 3-7 40 0.13 0.85 1.55 0.0046 0.032 0.019 0.0022 0.020 Inv. Ex. 3-8 40 0.09 0.15 0.90 0.0035 0.012 0.014 0.0021 0.45 Comp. Ex. 3-8 40 0.09 0.16 0.88 0.0036 0.013 0.012 0.0025 1.10 Inv. Ex. 3-9 40 0.12 0.33 0.81 0.0066 0.021 0.033 0.0045 0.21 0.15 0.023 Comp. Ex. 3-9 40 0.12 0.32 0.80 0.0256 0.023 0.035 0.0046 0.21 0.14 0.025 Inv. Ex. 3-10 100 0.14 0.05 0.40 0.0045 0.018 0.032 0.0035 0.43 0.45 0.025 0.008 Comp. Ex. 3-10 100 0.14 0.04 0.42 0.0046 0.019 0.032 0.0036 0.43 0.44 0.026 0.007 Inv. Ex. 3-11 100 0.10 0.35 0.65 0.0055 0.031 0.025 0.0035 0.40 0.45 0.025 Comp. Ex. 3-11 100 0.10 0.35 0.66 0.0056 0.042 0.033 0.0036 0.41 0.46 0.026 Inv. Ex. 3-12 100 0.08 0.45 0.55 0.0035 0.028 0.031 0.0037 0.68 0.45 0.056 Comp. Ex. 3-12 100 0.08 0.46 0.56 0.0036 0.029 0.120 0.0038 0.67 0.44 0.057 Inv. Ex. 3-13 6 0.08 0.35 0.85 0.0055 0.015 0.032 0.0039 0.22 0.23 Comp. Ex. 3-13 6 0.08 0.34 0.84 0.0054 0.016 0.032 0.0038 0.21 0.24 Inv. Ex. 3-14 6 0.13 0.25 1.21 0.0038 0.022 0.035 0.0036 0.15 Comp. Ex. 3-14 6 0.13 0.25 1.22 0.0039 0.023 0.036 0.0035 0.16

TABLE 34 (Table 33 Continuation) Ratio of micro Vickers Vickers Ferrite hardness in specific hard- Cu Ni B REM Ca Zr Mg fraction range (%) ness mass % X1 X2 % ≦190 HV ≦180 HV ≦170 HV HV Inv. Ex. 3-1 0.235 0.161 32 14 0 192 Comp. Ex. 3-1 0.274 0.158 29 13 0 205 Inv. Ex. 3-2 0.35 0.45 0.208 0.306 55 185 Comp. Ex. 3-2 0.34 0.46 0.206 0.295 0 188 Inv. Ex. 3-3 0.199 0.479 68 195 Comp. Ex. 3-3 0.210 0.053 33 189 Inv. Ex. 3-4 0.0025 0.0015 0.154 0.246 40 201 Comp. Ex. 3-4 0.0024 0.0018 0.166 0.291 48 215 Inv. Ex. 3-5 0.215 0.330 55 28 12 205 Comp. Ex. 3-5 0.216 0.329 19 10 0 215 Inv. Ex. 3-6 0.185 0.477 75 190 Comp. Ex. 3-6 0.176 0.451 73 163 Inv. Ex. 3-7 0.232 0.157 28 5 0 185 Comp. Ex. 3-7 0.233 0.110 21 3 0 196 Inv. Ex. 3-8 0.0023 0.170 0.533 55 188 Comp. Ex. 3-8 0.0025 0.213 1.286 31 220 Inv. Ex. 3-9 0.193 0.433 77 185 Comp. Ex. 3-9 0.192 0.430 75 182 Inv. Ex. 3-10 0.0008 0.217 1.663 55 185 Comp. Ex. 3-10 0.0007 0.217 1.567 20 182 Inv. Ex. 3-11 0.23 0.22 0.0023 0.211 1.069 35 16 3 178 Comp. Ex. 3-11 0.22 0.23 0.212 1.076 32 14 3 179 Inv. Ex. 3-12 0.190 1.809 67 188 Comp. Ex. 3-12 0.190 1.754 0 185 Inv. Ex. 3-13 0.155 0.476 72 188 Comp. Ex. 3-13 0.155 0.474 75 162 Inv. Ex. 3-14 0.211 0.165 50 185 Inv. Ex. 3-14 0.211 0.172 0 193

TABLE 35 Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 5′) Final plate thickness C Si Mn P S Al N Mo Cr Nb Ti V mm mass % Inv. Ex. 3-15 6 0.03 0.25 0.85 0.0038 0.015 0.033 0.0045 0.68 Comp. Ex. 3-15 6 0.03 0.24 0.86 0.0037 0.016 0.032 0.0115 0.66 Inv. Ex. 3-16 18 0.05 0.11 1.31 0.0025 0.021 0.031 0.0035 0.25 0.15 0.023 Comp. Ex. 3-16 18 0.003 0.12 1.32 0.0026 0.023 0.033 0.0036 0.26 0.15 0.024 Inv. Ex. 3-17 40 0.13 0.04 0.92 0.0066 0.013 0.025 0.0051 0.05 0.45 0.007 Comp. Ex. 3-17 40 0.13 0.03 0.91 0.0065 0.014 0.032 0.0050 0.05 1.15 0.008 Inv. Ex. 3-18 100 0.10 0.35 0.68 0.0055 0.020 0.033 0.0035 0.55 0.35 Comp. Ex. 3-18 100 0.10 0.34 0.67 0.0055 0.021 0.032 0.0036 0.54 0.36 Inv. Ex. 3-19 18 0.12 0.23 0.85 0.0063 0.031 0.023 0.0053 0.23 0.21 Comp. Ex. 3-19 18 0.12 0.22 0.84 0.0061 0.042 0.021 0.0051 0.22 0.22

TABLE 36 (Table 35 Continuation) Ratio of micro Vickers Ferrite hardness in specific Vickers Cu Ni B REM Ca Zr Mg fraction range (%) hardness mass % X1 X2 % ≦190 HV ≦180 HV ≦170 HV HV Inv. Ex. 3-15 0.115 0.459 55 35 25 201 Comp. Ex. 3-15 0.114 0.440 0 0 0 205 Inv. Ex. 3-16 0.143 0.265 43 178 Comp. Ex. 3-16 0.098 0.272 65 139 Inv. Ex. 3-17 0.203 0.308 52 175 Comp. Ex. 3-17 0.237 0.693 31 162 Inv. Ex. 3-18 0.200 1.169 50 188 Comp. Ex. 3-18 0.199 1.176 25 195 Inv. Ex. 3-19 0.196 0.448 40 188 Comp. Ex. 3-19 0.195 0.445 44 187

TABLE 37 Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 6′) Final plate thickness C Si Mn P S Al N Mo Cr Nb Ti V mm mass % Inv. Ex. 3-20 6 0.13 0.45 0.75 0.0068 0.015 0.033 0.0048 0.23 0.012 Comp. Ex. 3-20 6 0.13 0.44 0.76 0.0069 0.016 0.033 0.0049 0.24 0.011 Inv. Ex. 3-21 18 0.12 0.35 0.32 0.0035 0.022 0.016 0.0035 0.35 0.25 0.015 Comp. Ex. 3-21 18 0.12 0.36 0.31 0.0036 0.021 0.023 0.0036 0.36 0.24 0.015 Inv. Ex. 3-22 40 0.08 0.15 0.35 0.0050 0.018 0.030 0.0030 0.45 0.35 0.045 Comp. Ex. 3-22 40 0.08 0.16 0.36 0.0051 0.019 0.032 0.0031 0.46 0.36 0.105 Inv. Ex. 3-23 100 0.13 0.35 0.78 0.0045 0.025 0.031 0.0035 0.28 0.35 0.015 0.018 Comp. Ex. 3-23 100 0.13 0.36 0.77 0.0045 0.026 0.031 0.0036 0.28 0.34 0.015 0.017 Inv. Ex. 3-24 18 0.11 0.25 1.25 0.0032 0.022 0.031 0.0035 0.05 0.21 Comp. Ex. 3-24 18 0.11 0.24 1.24 0.0031 0.021 0.031 0.0036 0.05 0.22

TABLE 39 Plate Thickness, Chemical Ingredients, Etc. of Steel Plate (Method of Production 7′) Final plate thickness C Si Mn P S Al N Mo Cr Nb Ti V mm mass % Inv. Ex. 3-25 6 0.13 0.25 0.70 0.0035 0.018 0.031 0.0035 0.25 Comp. Ex. 3-25 6 0.13 0.26 1.10 0.0036 0.019 0.032 0.0036 Inv. Ex. 3-26 18 0.08 0.22 0.85 0.0035 0.025 0.031 0.0034 0.21 0.25 0.008 Comp. Ex. 3-26 18 0.08 0.21 0.86 0.0036 0.026 0.031 0.0035 0.22 0.26 0.008 Inv. Ex. 3-27 40 0.11 0.75 0.99 0.0045 0.023 0.025 0.0036 0.22 0.015 Comp. Ex. 3-27 40 0.11 0.76 0.99 0.0043 0.022 0.025 0.0036 0.21 0.016 Inv. Ex. 3-28 100 0.13 0.56 1.12 0.0044 0.032 0.024 0.0042 0.35 0.035 Comp. Ex. 3-28 100 0.13 0.55 1.11 0.0042 0.032 0.024 0.0043 0.34 0.110 Inv. Ex. 3-29 100 0.08 0.35 0.95 0.0035 0.031 0.031 0.0055 0.33 0.48 Comp. Ex. 3-29 100 0.08 0.35 0.96 0.0036 0.032 0.032 0.0053 0.32 0.46

TABLE 40 (Table 39 Continuation) Ferrite Ratio of micro Vickers hardness Vickers Cu Ni B REM Ca Zr Mg fraction in specific range (%) hardness mass % X1 X2 % ≦190 HV ≦180 HV ≦170 HV HV Inv. Ex. 3-25 0.190 0.429 50 178 Comp. Ex. 3-25 0.194 0.047 0 181 Inv. Ex. 3-26 0.156 0.446 62 185 Comp. Ex. 3-26 0.158 0.456 65 163 Inv. Ex. 3-27 0.199 0.374 45 182 Comp. Ex. 3-27 0.199 0.366 43 160 Inv. Ex. 3-28 0.232 0.413 55 178 Comp. Ex. 3-28 0.238 0.405 55 176 Inv. Ex. 3-29 0.0010 0.190 0.674 56 177 Comp. Ex. 3-29 0.0060 0.214 0.646 61 175

TABLE 41 Production Conditions (Method of Production 4′) Rough rolling Final total Finish rolling Slab plate Heating reduction T4- T4- 1st pass Total thickness thickness temperature rate X2 T4 40 80 bite reduction rate mm mm ° C. % — ° C. ° C. ° C. ° C. % Inv. Ex. 3-1 240 18 1020 85 0.161 906 866 826 875 50 Comp. Ex. 3-1 240 18 1020 85 0.158 905 865 825 878 50 Inv. Ex. 3-2 240 18 1100 85 0.306 928 888 848 855 50 Comp. Ex. 3-2 240 18 1100 85 0.295 927 887 847 945 50 Inv. Ex. 3-3 240 18 1150 81 0.479 944 904 864 861 60 Comp. Ex. 3-3 240 18 1150 81 0.053 867 827 787 860 60 Inv. Ex. 3-4 240 6 1120 88 0.246 965 925 885 952 80 Comp. Ex. 3-4 240 6 1120 88 0.291 971 931 891 950 80 Inv. Ex. 3-5 240 6 1000 88 0.330 976 936 896 900 80 Comp. Ex. 3-5 240 6 1000 88 0.329 976 936 896 705 80 Inv. Ex. 3-6 120 6 1150 83 0.477 989 949 909 880 70 Comp. Ex. 3-6 120 6 1150 83 0.203 959 919 879 865 70 Inv. Ex. 3-7 240 40 1080 67 0.157 853 813 773 825 50 Comp. Ex. 3-7 240 40 1080 67 0.110 840 800 760 828 50 Inv. Ex. 3-8 240 40 1200 67 0.533 896 856 816 830 50 Comp. Ex. 3-8 240 40 1200 67 1.286 926 886 846 850 50 Inv. Ex. 3-9 240 40 1230 67 0.433 888 848 808 795 50 Comp. Ex. 3-9 240 40 1230 67 0.430 888 848 808 792 50 Inv. Ex. 3-10 400 100 1180 50 1.313 835 795 755 810 50 Comp. Ex. 3-10 400 100 1180 67 1.245 833 793 753 815 25 Inv. Ex. 3-11 400 100 1000 50 1.069 828 788 748 765 50 Comp. Ex. 3-11 400 100 1000 50 1.076 828 788 748 768 50 Inv. Ex. 3-12 400 100 1250 38 1.809 846 806 766 750 60 Comp. Ex. 3-12 400 100 1250 25 1.754 845 805 765 755 67 Inv. Ex. 3-13 240 6 1100 79 0.476 989 949 909 895 88 Comp. Ex. 3-13 240 6 1100 79 0.474 988 948 908 898 88 Inv. Ex. 3-14 240 6 1150 75 0.165 952 912 872 943 90 Comp. Ex. 3-14 240 6 1150 75 0.172 953 913 873 942 90 Water cooling 1st half 2nd Flow End cooling half cooling Tempering rate temperature rate rate temperature m³/m² · min ° C. ° C./s ° C./s ° C. Inv. Ex. 3-1 1.0 451 — — — Comp. Ex. 3-1 1.0 455 — — — Inv. Ex. 3-2 0.3/1.5 213 4 25 580 Comp. Ex. 3-2 0.3/1.5 211 4 25 580 Inv. Ex. 3-3 0.7 120 — — 550 Comp. Ex. 3-3 0.7 125 — — 550 Inv. Ex. 3-4 1.0 105 — — 600 Comp. Ex. 3-4 1.0 120 — — 600 Inv. Ex. 3-5 0.5 222 — — 450 Comp. Ex. 3-5 0.5 225 — — 450 Inv. Ex. 3-6 0.5 550 — — — Comp. Ex. 3-6 0.5 620 — — — Inv. Ex. 3-7 1 235 — — 580 Comp. Ex. 3-7 1 221 — — 580 Inv. Ex. 3-8 1 105 — — 480 Comp. Ex. 3-8 1 115 — — 480 Inv. Ex. 3-9 2 465 — — — Comp. Ex. 3-9 2 450 — — — Inv. Ex. 3-10 2 205 — — 450 Comp. Ex. 3-10 2 210 — — 450 Inv. Ex. 3-11 1 20 — — 480 Comp. Ex. 3-11 1 20 — — 480 Inv. Ex. 3-12 3 20 — — 550 Comp. Ex. 3-12 3 20 — — 550 Inv. Ex. 3-13 1 150 — — 500 Comp. Ex. 3-13 0.1 130 — — 500 Inv. Ex. 3-14 1 160 — — 500 Comp. Ex. 3-14 6 155 — — 500

TABLE 42 Production Conditions (Method of Production 5′) Final Cooling Tempering Slab thickness plate thickness Reheating temperature Cooling rate end temperature temperature mm mm ° C. ° C./s ° C. ° C. Inv. Ex. 3-15 240 6 950 50 150 480 Comp. Ex. 3-15 240 6 950 105 155 480 Inv. Ex. 3-16 240 18 950 30 450 — Comp. Ex. 3-16 240 18 950 0.8 465 — Inv. Ex. 3-17 240 40 1000 20 450 — Comp. EX. 3-17 240 40 1000 20 550 — Inv. Ex. 3-18 400 100 930 3 135 450 Comp. Ex. 3-18 400 100 1100 3 155 450 Inv. Ex. 3-19 240 18 1000 15 205 500 Comp. Ex. 3-19 240 18 880 15 220 500

TABLE 43 Production Conditions (Method of Production 6′) Rough rolling Finish rolling Final total 1st pass Total Water cooling Slab plate Heating reduction bite reduction Start Flow rate End Tempering thickness thickness temperature rate temperature rate temperature m³/ temperature temperature mm mm ° C. % ° C. % ° C. m² · min ° C. ° C. Inv. Ex. 3-20 240 6 1100 88 885 60 720 1 201 550 Comp. Ex. 3-20 240 6 1100 88 875 60 640 1 222 550 Inv. Ex. 3-21 240 18 1150 81 860 60 713 1.5 425 — Comp. Ex. 3-21 240 18 1150 81 855 60 711 0.1 435 — Inv. Ex. 3-22 240 40 1120 58 908 60 721 2 55 550 Comp. Ex. 3-22 240 40 1120 79 910 20 735 2 78 550 Inv. Ex. 3-23 400 100 1180 50 753 50 711 1 405 — Comp. Ex. 3-23 400 100 1180 25 755 67 732 1 512 — Inv. Ex. 3-24 240 18 1100 81 925 60 715 0.7 215 500 Comp. Ex. 3-24 240 18 1100 81 920 60 790 0.7 210 500

TABLE 44 Production Conditions (Method of Production 7′) Finish rolling Final Rough rolling 1st pass Total Water cooling Tempering Slab plate Heating total reduction bite reduction Reheating Cooling End tempera- thickness thickness temperature rate temperature rate temperature rate temperature ture mm mm ° C. % ° C. % ° C. ° C./s ° C. ° C. Inv. Ex. 3-25 240 6 1100 88 880 80 750 30 20 500 Comp. Ex. 3-25 240 6 1100 88 885 80 710 30 20 500 Inv. Ex. 3-26 240 18 1150 85 912 50 770 0.5 460 — Comp. Ex. 3-26 240 18 1150 85 910 50 920 0.5 475 — Inv. Ex. 3-27 240 40 1200 67 945 50 750 10 450 — Comp. Ex. 3-27 240 40 1200 67 955 50 750 10 550 — Inv. Ex. 3-28 400 100 1150 50 865 50 740 2 20 450 Comp. Ex. 3-28 400 100 1150 68 862 23 740 2 20 450 Inv. Ex. 3-29 400 100 1100 50 811 50 770 5 20 500 Comp. Ex. 3-29 400 100 1100 25 810 67 770 5 20 500

TABLE 45 Results of Evaluation of Properties (Method of Production 4′) Tensile No. Yield stress strength Matrix toughness of holes MPa MPa (vE₋₅) J No. Inv. Ex. 3-1 475 612 71 345 Comp. Ex. 3-1 535 665 15 308 Inv. Ex. 3-2 468 613 88 665 Comp. Ex. 3-2 470 622 84 2 Inv. Ex. 3-3 505 635 135 845 Comp. Ex. 3-3 495 613 125 5 Inv. Ex. 3-4 482 623 41 322 Comp. Ex. 3-4 525 656 5 256 Inv. Ex. 3-5 515 656 48 655 Comp. Ex. 3-5 535 648 6 20 Inv. Ex. 3-6 475 595 94 885 Comp. Ex. 3-6 425 540 83 678 Inv. Ex. 3-7 485 601 70 302 Comp. Ex. 3-7 525 648 77 11 Inv. Ex. 3-8 468 596 100 525 Comp. Ex. 3-8 535 710 17 208 Inv. Ex. 3-9 505 595 185 972 Comp. Ex. 3-9 502 613 5 450 Inv. Ex. 3-10 475 585 87 225 Comp. Ex. 3-10 480 595 65 10 Inv. Ex. 3-11 465 578 78 178 Comp. Ex. 3-11 470 585 7 139 Inv. Ex. 3-12 505 603 88 261 Comp. Ex. 3-12 503 623 5 2 Inv. Ex. 3-13 525 635 92 683 Comp. Ex. 3-13 415 528 68 495 Inv. Ex. 3-14 475 623 78 256 Comp. Ex. 3-14 535 678 85 12

TABLE 46 Results of Evaluation of Properties (Method of Production 5′) Yield Matrix toughness stress Tensile (vE₋₅) No. of holes MPa strength MPa J No. Inv. Ex. 3-15 525 638 165 889 Comp. Ex. 3-15 523 610 10 5 Inv. Ex. 3-16 468 608 105 255 Comp. Ex. 3-16 329 445 88 189 Inv. Ex. 3-17 450 585 125 450 Comp. Ex. 3-17 417 555 15 208 Inv. Ex. 3-18 469 578 88 678 Comp. EX. 3-18 473 615 65 23 Inv. Ex. 3-19 512 623 78 315 Comp. Ex. 3-19 516 615 15 300

TABLE 47 Results of Evaluation of Properties (Method of Production 6′) Yield Matrix toughness stress Tensile (vE-5) No. of holes MPa strength MPa J No. Inv. Ex. 3-20 465 613 108 355 Comp. Ex. 3-20 413 556 83 218 Inv. Ex. 3-21 464 611 136 455 Comp. Ex. 3-21 395 535 125 389 Inv. Ex. 3-22 492 632 87 925 Comp. Ex. 3-22 512 625 25 15 Inv. Ex. 3-23 448 575 105 312 Comp. Ex. 3-23 420 555 20 256 Inv. Ex. 3-24 463 585 78 222 Comp. Ex. 3-24 462 611 85 11

TABLE 48 Results of Evaluation of Properties (Method of Production 7′) Yield Matrix toughness stress Tensile (vE₋₅) No. of holes MPa strength MPa J No. Inv. Ex. 3-25 480 612 78 315 Comp. Ex. 3-25 485 615 80 10 Inv. Ex. 3-26 463 573 80 450 Comp. Ex. 3-26 432 551 78 350 Inv. Ex. 3-27 505 605 66 380 Comp. Ex. 3-27 425 538 88 250 Inv. Ex. 3-28 511 638 68 350 Comp. Ex. 3-28 512 625 10 12 Inv. Ex. 3-29 476 595 90 225 Comp. Ex. 3-29 485 588 13 228

Invention Examples 3-1 to 3-14 are steel plates produced by the first method of production of steel plate described in (3) of the present invention (method of production 4′), that is, the method of water cooling speedily after rolling, and the method of production described in (14) of the present invention. Comparative Example 3-1 to 3-14 are also shown.

Invention Example 3-1 is plate thickness 18 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-1 is similar to Invention Example 3-1 in ingredients and method of production, but the amount of C and X1 are outside the range of the present invention, so the toughness is extremely low.

Invention Example 3-2 is plate thickness 18 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-2 is similar to Invention Example 3-2 in ingredients and method of production, but the finish rolling first pass bite temperature and the ferrite fraction are outside the range of the present invention, so the machineability is extremely low.

Invention Example 3-3 is plate thickness 18 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-3 is similar to Invention Example 3-3 in ingredients and method of production, but the X2 is outside the range of the present invention, so the machineability is extremely low.

Invention Example 3-4 is a plate thickness 6 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-4 is similar in ingredients and method of production with Invention Example 3-4, but the amount of Si is outside the range of the present invention, so the toughness is extremely low.

Invention Example 3-5 is a plate thickness 6 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-5 is similar in ingredients and method of production with Invention Example 3-5, but the finish rolling first pass bite temperature and the micro Vickers hardness are outside the range of the present invention, so the toughness and machineability are extremely low.

Invention Example 3-6 is a plate thickness 6 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-6 is similar to Invention Example 3-6 in ingredients and method of production, but the water cooling end temperature is outside the range of the present invention, so the strength is extremely low.

Invention Example 3-7 is plate thickness 40 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-7 is similar to Invention Example 3-7 in ingredients and method of production, but the amount of Mn and X2 are outside the range of the present invention, so the machineability is extremely low.

Invention Example 3-8 is plate thickness 40 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-8 is similar to Invention Example 3-8 in ingredients and method of production, but the amount of Mo is outside the range of the present invention, so the toughness is extremely low.

Invention Example 3-9 is plate thickness 40 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-9 is similar to Invention Example 3-9 in ingredients and method of production, but the amount of P is outside the range of the present invention, so the toughness is extremely low.

Invention Example 3-10 is plate thickness 100 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-10 is similar to Invention Example 3-10 in ingredients and method of production, but the total reduction rate of the finish rolling and the ferrite fraction are outside the range of the present invention, so the machineability is extremely low.

Invention Example 3-11 is plate thickness 100 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-11 is similar to Invention Example 3-11 in ingredients and method of production, but the amount of S is outside the range of the present invention, so the toughness is extremely low.

Invention Example 3-12 is plate thickness 100 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-12 is similar to Invention Example 3-12 in ingredients and method of production, but the amount of Al, total reduction rate of the rough rolling, and ferrite fraction are outside the range of the present invention, so the machineability and toughness are extremely low.

Invention Example 3-13 is a plate thickness 6 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-13 is similar to Invention Example 3-13 in ingredients and method of production, but the flow rate and Vickers hardness are outside the range of the present invention, so the strength is extremely low.

Invention Example 3-14 is a plate thickness 6 mm steel plate produced by the first method of production (method of production 4′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-14 is similar to Invention Example 3-14 in ingredients and method of production, but the flow rate and ferrite fraction are outside the range of the present invention, so the machineability is extremely low.

Invention Examples 3-15 to 3-19 are steel plates produced by the second method of production of steel plate described in (3) of the present invention (method of production 5′), that is, the method of reheating when the temperature of the steel plate falls after rolling, then water cooling, and the method of production described in (16) of the present invention. Comparative Examples 3-15 to 3-19 are also shown.

Invention Example 3-15 is plate thickness 6 mm steel plate produced by the second method of production (method of production 5′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-15 is similar to Invention Example 3-15 in ingredients and method of production, but the amount of N, the cooling rate of water cooling after reheating, and the micro Vickers hardness are outside the range of the present invention, so the machineability and toughness are extremely low.

Invention Example 3-16 is plate thickness 18 mm steel plate produced by the second method of production (method of production 5′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-16 is similar to Invention Example 3-16 in ingredients and method of production, but the amount of C, cooling rate of water cooling after reheating, and Vickers hardness are outside the range of the present invention, so the strength is extremely low.

Invention Example 3-17 is plate thickness 40 mm steel plate produced by the second method of production (method of production 5′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-17 is similar to Invention Example 3-17 in ingredients and method of production, but the amount of Cr, the end temperature of water cooling after reheating, and the Vickers hardness are outside the range of the present invention, so the strength and toughness are extremely low.

Invention Example 3-18 is plate thickness 100 mm steel plate produced by the second method of production (method of production 5′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-18 is similar to Invention Example 3-18 in ingredients and method of production, but the reheating temperature and ferrite fraction are outside the range of the present invention, so the machineability is extremely low.

Invention Example 3-19 is plate thickness 18 mm steel plate produced by the second method of production (method of production 5′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-19 is similar to Invention Example 3-19 in ingredients and method of production, but the amount of S and reheating temperature are outside the range of the present invention, so the toughness is extremely low.

Invention Examples 3-20 to 3-24 are steel plate produced by the third method of production of steel plate described in (3) of the present invention (method of production 6′), that is, the method of air cooling until the start of formation of ferrite after rolling, then water cooling, and the method of production described in (17) of the present invention. Comparative Examples 3-20 to 3-24 are also shown.

Invention Example 3-20 is plate thickness 6 mm steel plate produced by the third method of production (method of production 6′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-20 is similar to Invention Example 3-20 in ingredients and method of production, but the water cooling start temperature and Vickers hardness are outside the range of the present invention, so the strength is extremely low.

Invention Example 3-21 is plate thickness 18 mm steel plate produced by the third method of production (method of production 6′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-21 is similar to Invention Example 3-21 in ingredients and method of production, but the flow rate and Vickers hardness are outside the range of the present invention, so the strength is extremely low.

Invention Example 3-22 is plate thickness 40 mm steel plate produced by the third method of production (method of production 6′) It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-22 is similar to Invention Example 3-22 in ingredients and method of production, but the amount of V and the total reduction rate in the finish rolling are outside the range of the present invention, so the toughness and machineability are extremely low.

Invention Example 3-23 is plate thickness 100 mm steel plate produced by the third method of production (method of production 6′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-23 is similar to Invention Example 3-23 in ingredients and method of production, but the total reduction rate in the rough rolling, the water cooling end temperature, and the Vickers hardness are outside the range of the present invention, so the strength and toughness are extremely low.

Invention Example 3-24 is plate thickness 18 mm steel plate produced by the third method of production (method of production 6′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-24 is similar to Invention Example 3-24 in ingredients and method of production, but the water cooling start temperature and ferrite fraction are outside the range of the present invention, so the machineability is extremely low.

Invention Examples 3-25 to 3-29 are steel plates produced by the fourth method of production of steel plate described in (3) of the present invention (method of production 7′), that is, the method of heating again to the two-phase region after the temperature of the steel plate falls after rolling, and the method of production described in (18) of the present invention. Comparative Examples 3-25 to 3-29 are also shown.

Invention Example 3-25 is plate thickness 6 mm steel plate produced by the fourth method of production (method of production 7′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-25 is similar to Invention Example 3-25 in ingredients and method of production, but the X2, reheating temperature, and ferrite fraction are outside the range of the present invention, so the machineability is extremely low.

Invention Example 3-26 is plate thickness 18 mm steel plate produced by the fourth method of production (method of production 7′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-26 is similar to Invention Example 3-26 in ingredients and method of production, but the Vickers hardness and reheating temperature are outside the range of the present invention, so the strength is extremely low.

Invention Example 3-27 is plate thickness 40 mm steel plate produced by the fourth method of production (method of production 7′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-27 is similar to Invention Example 3-27 in ingredients and method of production, but the water cooling end temperature and the Vickers hardness are outside the range of the present invention, so the strength is extremely low.

Invention Example 3-28 is plate thickness 100 mm steel plate produced by the fourth method of production (method of production 7′). It satisfies all of the requirements defined in the present invention, so has 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-28 is similar to Invention Example 3-28 in ingredients and method of production, but the amount of V and the total reduction rate of the finish rolling are outside the range of the present invention, so the toughness and machineability are extremely low.

Invention Example 3-29 is plate thickness 100 mm steel plate produced by the fourth method of production (method of production 7′). It satisfies all of the requirements defined in the present invention, so has a 570 MPa or higher tensile strength and simultaneously exhibits an excellent machineability and good toughness. On the other hand, Comparative Example 3-29 is similar to Invention Example 3-29 in ingredients and method of production, but the amount of B and the total reduction rate of the rough rolling are outside the range of the present invention, so the toughness is extremely low.

From the above examples, it is clear that the steel materials produced by the present invention, that is, the steel plates of Invention Examples 3-1 to 3-29, are steel materials having a tensile strength of 570 to 720 MPa or so, a high matrix toughness, and an excellent machineability. 

1. Steel plate excellent in machineability and in toughness and weldability characterized in that the steel comprises, by mass %, a steel composition comprising: C: 0.005 to 0.2%, Si: 0.01 to 1%, Mn: 0.01 to 2%, P: 0.02% or less, S: 0.035% or less Al: 0.001 to 0.1% N, 0.01% or less, and the balance of iron and unavoidable impurities, X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less, when the plate thickness is 4 mm to less than 10 mm, a ferrite fraction of locations exactly ¼ and ¾ of the plate thickness inside from a top surface of the steel plate is 30% to 90% and a ferrite fraction of a location exactly ½ of the plate thickness inside from a top surface of the steel plate surface is 20% to 90%, and when the plate thickness is 10 mm to less than 100 mm, a ferrite fraction of a location 2 mm inside from a top and rear surface of the steel plate is 30% to 90% and a ferrite fraction of locations exactly ¼, ½, and ¾ of the plate thickness inside from a top surface of the steel plate surface is 20% to 90%.
 2. Steel plate excellent in machineability and in toughness and weldability, characterized in that the steel comprises, by mass %, a steel composition as set forth in claim 1 wherein: Mn: 0.01 to 1.4%, S: 0.01% or less, X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less, X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is 0.15 to 10.0, the structure forming the steel is a structure having a ferrite fraction of 30% to 90% and the balance of a hard structure mainly comprised of bainite and martensite or a structure having a ratio with a micro Vickers hardness of 190 HV or less of 20% or more, and the steel has a Vickers hardness of 165 HV to 300 HV.
 3. Steel plate excellent in machineability and in toughness and weldability, characterized in that the steel comprises, by mass %, a steel composition as set forth in claim 1 wherein: Mn: 0.01 to 1.4%, S: over 0.01% to 0.035%, X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less, X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is 0.15 to 10.0, the structure forming the steel is a structure having a ferrite fraction of 30% to 90% and the balance of a hard structure mainly comprised of bainite and martensite or a structure having a ratio with a micro Vickers hardness of 190 HV or less of 20% or more, and the steel has a Vickers hardness of 165 HV to 300 HV.
 4. Steel plate excellent in machineability and in toughness and weldability as set forth in claim 1, characterized in that said steel further comprises, by mass %, one or more of: Mo: 0.01 to 1%, Cr: 0.01 to 1%, Nb: 0.001 to 0.1%, Ti: 0.001 to 0.1%, V: 0.001 to 0.1%, Cu: 0.005 to 1%, Ni: 0.01 to 2%, B: 0.0002 to 0.005%, REM: 0.0005 to 0.1%, Ca: 0.0005 to 0.02%, Zr: 0.0005 to 0.02%, and Mg: 0.0005 to 0.02%
 5. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 1 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T1(° C.) expressed by T1=35 ln(X2/2)-25√t+1070, X2=(Si/5+Mo+Cr/2)/Mn, to 720° C. and a total reduction rate of 30% to 95%, starting water cooling after the end of the rolling at a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.
 6. A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in claim 5, characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.
 7. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 1 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it, starting water cooling when the steel plate surface temperature is T2(° C.) expressed by T2=910−310×C−80×Mn−20×Cu−15×Cr−55×Ni−80×Mo+0.0006t2−0.56t−8.6 to 650° C. by a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 500° C. or less, where t is the plate thickness.
 8. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 1 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, air cooling to 500° C. or less, reheating the steel plate to a T3(° C.) expressed by T3=0.0017t²+0.17t+730 to 850° C., then starting the water cooling, and ending the water cooling at 500° C. or less, where t is the plate thickness.
 9. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 2 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T4(° C.) expressed by T4=35 ln(X2/2)-25t+1100 to an Ar₃ point by a total reduction rate of 30% to 95%, then speedily starting water cooling after the end of the rolling at a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.
 10. A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in claim 9, characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.
 11. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 2 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rolling it, cooling it to 500° C. or less, reheating the steel plate to 900° C. to 1050° C., water cooling it by an average cooling rate of 1° C./s to 100° C./s, and ending the water cooling at 500° C. or less.
 12. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 2 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to an Ar₃ point to a temperature lower than the Ar₃ point by 150° C., starting water cooling at a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 500° C. or less.
 13. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 2 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to 500° C. or less, reheating the steel plate to 730° C. to less than 900° C., then water cooling it, and ending the water cooling to 500° C. or less.
 14. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 3 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T4(° C.) expressed by T4=35 ln(X2/2)−25√t+1100 to an Ar₃ point by a total reduction rate of 30% to 95%, then speedily starting water cooling at a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.
 15. A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in claim 14, characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.
 16. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 3 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rolling it, cooling it to 500° C. or less, reheating the steel plate to 900° C. to 1050° C., water cooling it by an average cooling rate of 1° C./s to 100° C./s, and ending the water cooling at 500° C. or less.
 17. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 3 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to an Ar₃ point to a temperature lower than the Ar₃ point by 150° C., starting water cooling at a flow rate of 0.2 m³/m²·min to 5.0 m³/m²·min, and ending the water cooling at 500° C. or less.
 18. A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in claim 3 and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then cooling it to 500° C. or less, reheating the steel plate to 730° C. to 900° C., water cooling it, and ending the water cooling at 500° C. or less.
 19. A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in claim 5, characterized in that said steel slab or cast slab further contains, by mass %, one or more of: Mo: 0.01 to 1%, Cr: 0.01 to 1%, Nb: 0.001 to 0.1%, Ti: 0.001 to 0.1%, V: 0.001 to 0.1%, Cu: 0.005 to 1%, Ni: 0.01 to 2%, B: 0.0002 to 0.005%, REM: 0.0005 to 0.1%, Ca: 0.0005 to 0.02%, Zr: 0.0005 to 0.02%, and Mg: 0.0005 to 0.02% 